Looking for some advice from someone(s) who has been there and done that in this situation.

We just picked up an older 47cm (1998, 1999?) Cannondale Cad2 R300 w/650c wheels for my girlfriend's first road bike. It's equipped with Shimano RSX (3x7) brifters and derailleurs. The bike is solid but the rear wheel has a dent in the braking surface and is just generally worn out.

We want to buy a new wheelset. Hopefully something we can upgrade to 10 speed later on as I upgrade from the 2009 Shimano 105 components on my bike. Thankfully the hub spacing on the rear is 130mm, which should make the frame compatible with the current wheelsets for 8/9/10 speeds if I'm not mistaken.

What we'd like to do is buy a new 8/9/10 speed compatible wheelset and use the current HG50 7-speed cassette for now and move the 105 group over to this bike once I upgrade. My understanding is that we may have to buy a 4.5mm spacer if one isn't already on the bike. My guess is that with the 130mm hub spacing, there is already one present.

Am I on track? Am I missing something? Am I headed for disaster? Any feedback from folks that have some experience in this would be very much appreciated.

Oh, and, second question. Since the shift indexing occurs at the shifter and not the derailleur, is it possible to use 10-speed Shimano 105 brifters and NOT change out the old RSX derailleurs? My thinking is that the derailleurs still have the same overall range of movement even though the increments will be much smaller. I know it seems a little silly to swap out the expensive parts but not the inexpensive ones, but it would be nice to know in case we can't budget for the derailleurs right away.