Gareth, dropping running gear into a cheap Stagea is straight forward. Same issues with dropping into a GTSt regarding pipwork and the added issue of converting to manual. Seems straight forward enough though. No need to cut holes in the firewall, etc. It all bolts over.
Mike is a dirty wheel whore
That will either be on a RS4 S or an Autech. I run 18 x 9.5 +22's and in the rear of my auto, they don't fit unless you run about 3 degrees of neg camber.
Darren is on the money with the rear cradles. You can bolt in a cradle from a S14/GTR/etc, with the best bet going for an Aus S14/15 rear end as they don't have Hicas.
To start off with, I didn't really like the S2's however, the front end grew on me and I ended up buying one. For a stocker that you don't intend on buggerising around with or you want to keep it close to stock (ie, boost, turbo, cooler, etc) the S2 is a winner.
Neo motor, tiptronic, if you're lucky to find one like mine, it'll have leather/electric/heated seats, touchscreen navi/climate control etc.
Considering you want to make a monster out of a Volvo Hurse, then I would go for an S1. They are cheaper to buy, parts are cheaper than S2 parts (panels, trims, etc) and you'll be wanting to cut it up anyways.
I found Chris and Min's Stag for around $6800-ish 3 years ago. Something like that now would probably go for around 5k. Leaves 5k to spend on go fast bits if your budget is 10k.
I found mine at auction and got Aaron to import it, etc and complied and on the road it cost me around $13,000. This is what it looked like at auction.