I would take the advice on the gearbox! The sillyspeed is so called for a reason! Works well when looked after but has its known issues and can be an expense at times, but your on the right forum for all the advice and help you could wish for!
Thankfully i bought my 147 with the kit already on it but i believe the going rate is around £1200 or so before painting for all the bodykit parts, check out autodelta's shop! worth a good look especially if you like modifying an already great car. (my 147 has their 205bhp turbo conversion)
With regards to the one your looking to buy, seems around market value really (but as always there will be room for bargaining!), also check date of timing belt change, should be three years or the 36,000 miles whichever is sooner, just saying as my 147 did 149 miles in 4 years before i bought it, so even though it was still a new belt basically, no chances were took and it was off the car immediately, better a few hundred quid for the belt than a four figure sum to rebuild after belt snap.