I think that is a good price and you should get some interest on the wheels. I have had to put off a long-standing build idea that I was thinking of using them for I'm afraid. If they haven't gone soon though drop me a pm and I might go for them anyway (monitoring finances closely at the moment).
I am definitely interested in your Thomson stem if still available. Although its not retro it will suit my p7 fine (originally built for the wife then discovered its about the right height for her but too stretched even with short stems).
Oh, and on the xt rear mech question, is it long cage (sgs) mech or short (gs) ?
If the former I would be a bit surprised if it didn't work ok with 7-speed. Its the shifters that largely determine the horizontal position of the mech over the cassette. I think its ok to use mechs across different speed setups e.g. I've used 9-speed in 8-speed setups ok.
The issues to check though are as follows.
1 Can mech cope with the range of gears that it will be used with (difference in number of teeth) ? - If long cage mech, then answer is nearly always yes.
2 Don't mix mechs and shifters from different manufacturers as actuation ratios may screw things up (with just a few exceptions), so this mech is ok if used with shimano shifters.
3 The correct speed chain must be used for the cassette as chain widths differ between setups. Also the chain must be an acceptable width for the mech. I've never had this cause a problem, but best thing is to check the width of the new mech cage against the old one being replaced.
4 The different number of teeth on jockey wheels can sometimes make setting the b-tension adjustment screws tricky when a mech is used in different speed setups than it was designed for, but I have managed to use 8 speed mechs in 7 speed setups before and 9 speed mechs in 8 speed setups.
Hope this helps.
Could always check by asking in the "chat" section.