As above, forged pistons are around £4-500 - if you could get the engine for under £1500 and were willing to strip & rebuild, testing everything properly and installing forged pistons, I think you'd be ok.
Also check the exact specs of the cylinder head, a lot of the crossflow community refer to 'big valve' heads as an upgrade, they're just a 1600 GT head and the valves are only fractionally larger than std, they're not actually a worked head. A properly sorted head will have the ports ground out extensively, which also involves machining down the bronze guides, something few builders bother to do but makes a huge difference. If that has been done, and it has larger valves installed, it's probably worth £1000 on its own (cost to get a head and have an engineer install valves, hardened seats for unleaded and the machining/flow work).
I'd still be cagey about what you could end up with though, if the blocks been decked too far already or corners have been cut, you could easily find yourself putting another £1500 into it.
HTH!