Scott and Marins are good solid makes and can be had for bargain prices, trouble with second hand MTB's though, even ones only a few years old let alone the ages of the bikes on here is that they tend to suffer from neglect, worn out chains and gears are common so you may need to budget for giving the bike a good service and check over to get it working properly unless you are lucky and get a bike that has been well maintained or kept indoors and hardly used, you stand a better chance of buying a bike like that on here than on E-bay TBH.
If you are going to build a bike i'd suggest going for a common popular frame like a Kona or Orange, that way you can ask on here the bottom bracket size and seatpost size needed etc and get positive answers, go for an obscure or less common frame and you may be left guessing.
In my experience the main advantage with discs is low and easier maintainance and better performance in the wet and your rims dont wear out , a well set up V-brake system performs just as well in the dry but are trickier to set up, i have one bike with discs but if i'm building a bike i stick with rim brakes.
I am looking on ebay and on here i would prefer to buy one of someone on here as what i have seen the bikes on here are well looked after.
Disc brakes sound ok to look after then, i hope to find something.
I see lots of you guys with bikes your building maybe someone is building something just right for me. lol