[Ains.]

Yep TP, agree with all that Jools has put above.
Prices roughly:

Early 851 1.8k upward
Late 851 (has the 888 bodywork with 851 stickers) 3.5k
888 strada 4-5k
888SP2-3 5-6k
888SP4 7-9k
888SP5 7-8k
888SP4S Cherching

They are going up in price.
Strada is the better road/all round bike. SP is nowt below 4krpm, a bang at 6k, bigger bang at 8k, and stupid by 10k.
I wouldn't get an SP unless you're going to look after it as the only ones a round now tend to be mint condition and fetching loadsa dosh. For instance a US modded SP4 went for about $26k, but it was spesh mind. The top end ones are going for basically what the buyer will pay to get one.
With a little effort a Strada can be made as quick as an SP. Without the layout of an SP so is a better bet. GazB's 851/916 is proof. The usual stuff, filter, zorst, chip.
Gofasterbitz are common enough, but as Jools said take the original bodywork off and stick GRP on for track work. Track day damage as long as the bike doesn't flip are panels, zorst casing, indicators, bar ends and scuffed leavers. Yup I threw the SP4S up the track at Cadwell in 2003 - I cried.
The fairing to frame supports are cack and need to be beefed up and fit a bigger brake master cylinder. That'll solve most of the brake problems.

Oh, and a few years ago Mark Forsyth in finishing off an article about the 888 said, the 888 is the biking equivalent of the Ferrari 250GTO, and that dear reader is why you want one.

Ains.