The appeal of Captain Fantastic is in the graphics more than the game play (IMHO). Bear in mind that's coming from a very biased single-player Gottlieb EM fan.
(But I have owned and restored Capt. Fantastic too.)
That said, the primary value of an EM is in the cosmetic condition, and specifically the backglass, playfield, and plastics. Repros are available, but expensive. If any of those elements are in poor condition, the value of the game will be very limited.
The backglass comes in two varieties: a later one that has some 'stars' strategically placed to cover some possibly offensive theme elements, and an earlier one without the stars (I'm sure you can find the differences on ipdb.org or such). Some folks place a premium on the 'non-stars' variety.
Decent to better cosmetics, shopped and working CFs generally sell in the $1000 range (800-1200). Project games needing attention and parts would be less than that (about half). They made a lot of them, and the supply generally meets the demand/interest fairly well. (They sold for around $1000 back in the 90's even, so haven't really kept pace with inflation.) Whether it works or not doesn't really matter so much, since the value is really in the cosmetic condition. From sitting in storage for so long, it will need a complete going through, rebuild, or at least a good shop job to ensure full and fresh operation. While it may function, it won't be in as good of operational shape as it can be. So anywhere from $100 to $750 could be 'fair', all depending on the cosmetics (and completeness; don't want any major parts missing). Look on pinpedia or bostonpinball.biz for example selling prices.
Before plugging in and turning on any EM, give it a thorough inspection, starting with the line cord, moving to the transformer, bottom board and so on. Check to make sure all the fuse are present and are the *correct* values (I've yet to find a game 'in the wild' that has the correct size fuses in it.) The correct fuses are there for protection, so may as well take advantage of them.
If the game has been sitting for several years, then the moving parts in things like stepper units are likely gummed up or seized up. Anything that is meant to move or turn should be examined and made to operate correctly mechanically. Also, all the contact points may have oxidized and will need burnishing for proper operation.
The decision to plug it in an turn it on really depends on the initial condition. If it's been sitting in a barn, shed, garage, and the inside is really dirty, corroded, etc., I wouldn't even consider plugging it in until after having done a complete tear down and rebuild of the game first. If the game's been in climate controlled storage, is generally clean, and just hasn't been used, then after giving it a review of the critical components, the fuses, wiring, and making sure stuff's not gummed up, then powering it up isn't such a big deal.
Parts are readily available, yes.
If you're not planning on doing all the refurbishing work yourself, then yes, it will be a money pit. If the game's generally pretty clean and just needs a good going through, clean, wax, new rings and bulbs, it's like $50-$75 in parts, and you do the labor. If you hire someone to do just that much, it will be more like $250+. If it need more extensive troubleshooting and repair, then figure a few hundred dollars. Hiring work done on EMs beyond simple one-time fixes is a money losing situation for most EMs (which is why it's a terrific hobby to collect and restore them on your own).
Major red flags would be things like repainted cabinets, touchups, clear coats, etc. - anything that would alter the originality of the game and excellent original cosmetics. There are too many CFs out there to bother with poor condition or modified ones.
If you're not into EMs, don't want to work on them and stuff, even 'free' isn't always a good deal. But picking up a CF for 300-400 bucks, spending a few weeks refurbishing it, and then playing it a while, maybe selling it down the road for 800-1000 is fun.
Good questions to ask though. Good luck!