Depends what it's already had done.
First up, £650 is a bargain, even if the shell is dead (provided you're prepared to re-shell it) as a G40 engine and box are worth a grand alone. Interiors go for a further £200.
If it hasn't already had it done, get the charger overhauled. It looks as if it's been owned by an enthusiast (albeit one with an odd taste in steering wheels!) so it's probably had it done at some point but it could probably do with another. Remember this is advisable every 30,000 miles.
As it's been sat around for a while, pray that moisture hasn't got into the charger as it had on mine, that can corrode the internals. Because doing so involves checking the condition of the unit, it could save you from suffering a dead charger which can happen if you run one that's not right. It could well highlight a problem with it that you didn't know existed. Worth looking into!
Other than that check the chassis legs for cracks and make sure the shell isn't bent. Looks like a good shell though, as long as the damage from the rear beam isn't more than it looks.
Unless it's a completely knackered shell, with collapsed seats, a dead engine, charger and gearbox then for £650 it's a bargain!

No seriously, if the worst comes to the worst and the shell is dead I can't see it costing much to find a mint condition 1 litre and swap everything over.
Hope this helps, and good luck!