2 years ago I paid 8k euros for a '02 (so 4 yrs old at the time) 2.4 JTD 156 SW. Aside from my wedding, this is probably the best 5,000 quid I ever spent! (watch it break down spectacularly on the way home now I've said that...

)
Prices across Europe vary a lot and I know that residuals are different in the UK from belgium so what I say should be taken with a large pinch of salt but I rekon you've got the right amount of money to get a nice one.
I know very little about cars (beyond an engineer's natural curiosity) and hadn't found AO at the time. I paid 5 GBP (iirc) for a 156 buyer's guide from an advertiser on the Top Gear website. No substitute for real expert knowledge I'm sure, but it gave me a few pointers to look at, mostly generic stuff like checking for even tyre wear, loose steering etc but some model and engine specific stuff aswell.
have fun and good luck! May the cloverleaf be with you....