Nothingman said:so looking at a around $400 bike from kmart or ebay is a waste of time?
not even worth thinking about, just junk for the tip?
my old bike has rockshox indy s forks. i could restore this bike, but would need a fair bit of work. chain, cables, tyres, back derailer is damaged. then who knows what else isnt working...
what about a second hand bike? or an old years model...
or should i be looking at a decent hard tail?
or is fixing up my mongoose the best bet?
Generally, I think eBay is great ... that is, if you have already found that mail order (in the past) was as good-or-better than your LBS based on a combination of product availability, price & service (many of the traditional mail order retailers like COLORADO CYCLIST & EXCEL provide fairly strong technical support) AND if you can do your own wrenching.
I still only have a hardtail ... but, have been watching-and-monitoring the full suspension market for many years because I have been trying to justify a full suspension bike (too?!?), but (fortunately?) can't (because, like electronic components, each year that passes has yielded more-bang-for-your-buck and a mid-range MTB is probably vastly superior to a high-end MTB from a half dozen years ago) ...
I think that if you can bump your target price UP a bit (closer to $600), you will PROBABLY get a much better bike (one that retails for $900-$1200) than if your budget is limited to $400 (at which point you will be get the equivalent of the lowest grade, entry level LBS bike).
Now, it is possible that the difference in performance will be negligible, but the weight difference will probably be 5+ lbs ... almost EVERYTHING on the new-but-last-year's-model bike @ $400 will probably be steel. So, it will be a glorified KMart bike at 2x the price (over 3x at LBS retail).
Of course, I know one person who replaced his old Specialized full suspension bike with a Santa Cruz which was easily ONE SIZE larger than I think he should have selected (he's about 5'10" & bought what has to be a LARGE frame) -- about the same amount of seatpost was exposed on his "new" MTB as on my ROAD bike! IMO, that kind of sizing is okay for fire roads & bike paths, but probably not for (semi-)technical riding ...
The frame size is critical ... when in doubt, opt for the ONE-size-smaller.
FWIW. And/(So), the matter of hardtail vs. full suspension depends on the type of riding you anticipate doing in the future OR if you found that the trails you rode in the past were more technical than you were comfortable with on your hardtail Mongoose.
AND, the TYPE of suspension linkage is important ... perhaps, not as much as a half-dozen years ago because of the improvement in the rear shock, but still important ...
SO, if I were in-the-market to replace my hardtail with a full suspension bike AND had a limited budget (as most of us do have!), then I think KONA and GT (and, the Schwinn MTBs which now incorporate the GT i-drive rear suspension linkage) have reasonably nice bikes that retail for under $1000US ... so, $600 as last year's model (or, a two year old remnant on the showroom floor) is reasonable at your LBS OR on eBay.
BTW. I tried a GT i-drive MTB a half dozen (!?!) years ago, and I was pretty much sold on THAT linkage design ("zero" bobbing ... an important aspect when you are going UPhill), but there are a few others I might/would consider now since rear shocks (FOX, in particular) are much better. The current i-drive frames are more "mechanic" friendly than the earlier design (that is, don't buy a MTB with an older, "hydro-formed" i-drive frame).
As far as your Rock Shox Indy fork (which I think ONLY uses Neoprene internals -- nothing wrong with that under limited conditions), if you aren't riding in cooler/colder weather & you don't need more than 83mm (?) of travel, then it can probably be brought back up to spec by putting some ENGLUND's SLICK HONEY on the fork's sliders.
SLICK HONEY is, by my reckoning, simply motor oil & vaseline -- that is, you can re-create something that looks-and-feels like SLICK HONEY by blending the two to a viscosity similar to [you guessed it] honey.
Generally, grease (real & elbow) & oil go a long way to rejuvenating many moving parts on a bike.
The rest of the parts, as garage sale GT has indicated, can be replaced for a reasonable amount ... particularly, if you are "wise shopper" on eBay.
If, after you "fix" your Mongoose, you feel a need for a different fork, then I recommend Marzocchi, first; Rock Shox, second. You can generally forget the others ...