Okay... late better than never!
First, this is a Firelight 12, the predecessor to the 600. If you search online, you will find the manual as a PDF from Jotul's site, along with an exploded parts diagram on several sites (I think the best resolution one is at woodmanspartsplus.com).
All stoves are a compromise, each having its own pros and cons inherit to the design, as well as those associated with age, parts availability, etc. So, here's the list of pros and cons I can list for the Firelight 12, based as much on reading what many others here have posted, as my own experience:
Pros:
1. It is a true gentle giant. It is a MONSTER heater, being one of the largest stoves on the market, but is also stated to be perhaps the most controllable stove ever operated by many of its owners. I can say the stove is very controllable, but I do not have experience with other stoves to which to compare. However, several Firelight 12 owners on this forum are also owners of Woodstocks, PE's, and even modern non-cat Jotuls. Just about every one of them has stated either publicly or in PM's that their Firelight 12 was their most infinitely controllable and well-behaved stove.
2. It is built like a tank. I do not think this is unique to the Firelight 12, but really all Jotul product. It is superbly designed and manufactured.
3. Beautiful. This is somewhat objective, but most will agree the Firelight (and really the whole Jotul line-up) is among the most attractive stoves ever built.
4. Long burn times... common to any catalytic stove, but worth pointing out, as the Firelight 12 is superior to the modern Firelight 600 (and really any non-cat stove), in this regard.
5. Even the senior support tech's at Jotul, who have been there long enough to be familiar with this old hunk of iron, will tell you this is one of the best stoves Jotul ever made.
6. Top-load or front load. Some people downplay this, while simultaneously touting the reasons they love their side-loader. Bottom line, top-loading enables you to get WAY more wood into a downdraft stove (cat is in the back, so upper firebox is completely free) than front or side loading alone. I use my front doors a good bit during shoulder season, but almost exclusively use the top-load door when the stove is burning 24/7. It eliminates any issue with piled up coals spilling out the doors (not uncommon for me to have a 6" - 8" deep coal bed when it gets blistering cold), and the foot pedal to open the door really is a nice convenience.
Cons:
1. It is an old stove, so manufacturer and dealer support is waning. You may luck out and find a dealer or support tech who has been working long enough to have experience with this stove, but it is becoming an issue for those seeking direct manufacturer support.
2. It is an old stove, so the cost of replacement parts is going up each year, as Jotul's cost to manufacture and store decreasing lot sizes goes up. I do not imagine any time in the near future where you will not be able to find parts for this stove (old Firelight 12's are plentiful on ebay and Craigslist), but you can't just walk into a dealer and have a new lid casting delivered overnight, either.
3. It is an old stove, so you will be rebuilding it, at some point. However, given the conditions you already outlined in our OP, that's a given!
4. Catalysts... this item has been troubling several of us Firelight 12 owners. The Steelcats recently promoted by Condar and a few others do not hold up to use in this stove, or most of the older downdraft catalytic stoves, it would appear. No trouble, the traditional ceramic cats work GREAT in these stoves! The trouble is that the two most popular aftermarket cat's that fit these stoves are constructed from a ceramic honeycomb wrapped in a steel "can". The idea is that the can holds the cat together, when the ceramic inevitably develops fissures. This is no big deal, and nothing new, except that the gasket material these two manufacturers are now inserting between the ceramic honeycomb and the steel can seems to be an expanding material, which may damage the soft refractory housing which Jotul uses to hold the catalyst. Several of us are experimenting with alternatives here, and would be happy to welcome another scientist to the group!
So, if this stove was in good operable condition, I would not hesitate to recommend it. I think the Firelight 12 is one of the best stoves ever built. So much, that I have recently acquired a THIRD (yes... I own three). Money is not an issue for me, and I could have easily bought a new stove, but I do not believe there is any new stove on the market with which I would be as happy as my F12's (although the BK Ashford does tempt me...).
Used F12's in good condition will sell for $800 - $1200, with a few exceptionally pretty or recently rebuilt examples fetching $1500. This stove sold for more than $3000 new. However, you do need to consider the cost of parts. Figure $75 for the inner rear burn plate, and $125 for the larger rear burn plate (confirm you can still buy it from Jotul!). Figure $450 for a new cat chamber (if damaged), and $70 for each of the two smaller refractory pieces (the cover will almost always be damaged, and the little piece that sits on the floor is often lost on used stoves I have inspected).
The damage you describe is common to any overfired F12. If the bypass damper assembly appears to be straight and functional, and the damage appears to be limited to the rear burn plate(s), this might not be a bad deal. I would try very hard to be sure the outer shell is intact, with all seams well-sealed, as an interior rebuild is MUCH simpler than disassembling the outer shell. The best way to do this is to remove the inner rear burn plate (small casting with fan pattern, held in with one bolt at the top), and the two inner side plates (they just lift out), and place a bright light inside the stove. Turn off the lights in the room and inspect everywhere outside for light at the seams. Then repeat with your head in the stove, and someone moving a bright light around the outside of the stove.
Are you someone who doesn't mind working on something like this? Many here have rebuilt F12's, most of us just the interior, and it's not a very difficult job. However, do mind begreen's advice, if you're not someone who likes to tinker. Any old stove can come with hidden issues, that may frustrate you down the road.