Bicycle Mechanics - 12 speed sti compatibility question

hi there, i have a Maruishi Rx-7, 12 speed, double chain ring, downtube shifters. it's apart for restoration, and i'm replacing a few parts, and i only want to spring for one good pro build once i purchase all the parts i want, i was wondering if it was possible to put STI on this bike? would a 5 speed cassette effectively render this bike 10 speed, and able to take 10 speed shimano sti? i'm looking at a set in the $175 dollar range on ebay. level with me, even if possible, is it worth it to go sti on vintage cromo frame bike? http://i174.photobucket.com/albums/w84/green814/Selling/DSC00605.jpg
note, not my bike, but same model. mine is a little beat, but being fixed up.

No, 10 speed means 10 cogs in back. It's the spacing and number of the cogs that make it compatible. You could respace your frame to 130mm, get a new wheelset that accepts a cassette, put a 10 speed cassette on, get a new chain, new cranks (new bottom bracket to support it), and obviously the brifters.

By my, and many others calculations, done right (decent components, not half-assing the work), you would be looking at a thousand dollar investment, assuming you do the work yourself. Lots of people have done it, but unless you really really really like your bike, or find some amazingly good deals, you may be better off getting a new bike.

So, is it worth it? That's a call only you can make . . .

So, is it worth it? That's a call only you can make . . .

Incorrect. I'll gleefully make that call.
No.
And I didn't even break a sweat.

ok. this was the kind of info i was looking for. i'll enjoy this bike with downtube shifters and save up for a more modern roadie. i got into roadbikes recently, from fixed and singlespeed riding, and have been cutting my teeth on vintage finds. looking forward to someday soon having a legit new roadie. any entry level recommendations?

It looks like the bike is SIS compatible (Shimano Indexed System rear derailer) so an STI conversion can probably be done.

If the rear hub is a cassette hub, it is Uniglide (there never was 6-speed Hyperglide) and you can't really find 7-speed Uniglide cassettes anymore. You could convert to a Hyperglide freehub, though, for about $20 or $30. You would then need a 7-speed Hyperglide cassette.

If it's a freewheel hub, get a 7-speed Shimano freewheel. They are still made but selection isn't the greatest.

Get Shimano Sora 7-speed STI shifters. These seem to be out of stock everywhere I've looked so you are SOL on this conversion unless you can find them http://www.bikeparts.com/productsearch.asp?categoryName=Shifter/Brake%20Combos&manufacturerName=&selManufacturer=&selCategory=1532&strSearchString=x7sp&displayName=7-Speed%20Sets

ok. this was the kind of info i was looking for. i'll enjoy this bike with downtube shifters and save up for a more modern roadie. i got into roadbikes recently, from fixed and singlespeed riding, and have been cutting my teeth on vintage finds. looking forward to someday soon having a legit new roadie. any entry level recommendations?

That's a good decision. Ride it as is and save for a new road bike.
A good fit is the most important part of buying a bike.