I'm with bikehard. If the disc adaptor is rare I'm guessing it will sell for $30-$50 so now you could be up to $250 for a frame that is almost 10 yrs old and not exactly suited to your needs. You may also need a different front derailleur for a FS rig so add in the possibility of another $30-$50. Seat tube may be a different size as well so throw in a seatpost and clamp. Sometimes you even have to screw around with the bottom bracket to get the chain to run straight (chainline).
Also: Going from a HT to FS usually means you'll need new cable housings (I've only gotten lucky once and have built several bikes) and even longer brake/shifter cables. This may sound like no big deal but trust me by the time you get cable housings, cables, cable ends, housing ends, zip ties, maybe a cable guide for the bottom bracket, misc clips, cable lube $10, etc you've spend a substantial amount of cash. Friend of mine somewhat recently went in to get these exact parts plus a few other misc items like handlebar grips and he ended up with a bill of almost $70!!! Lets just say he was pissed. His super cheap frame wasn't so cheap anymore.
Just to give you an idea I sold my 2004 Giant NRS 1 frame a year ago with a newer Fox shock (with lockout) for only $300. Hope we're not raining on your parade? Just want to give you as much info as possible so you don't make the same mistakes some of us have. Of course these mistakes are the best way to learn a lot about bikes. It however is very disappointing to start building the frame up only to discover that you've run into issues towards the end that will keep you broken down for a week while parts are on order. I've learned to NEVER tell anyone that you'll meet them on the trails later today soon as you put your new frame together.