New one = Â£475 + new clutch (because) Â£70-100 + some gaskets for fannymolds and the like.
Secondhand one = Â£200 + delivery/fetching (call it Â£50) + new timing belt kit (Â£90ish) + clutch (because engine out) plus gaskets since Clio sourced.
Rebuild what I've got = Â£90 gaskets + Â£90 timing belt + chances of it all just going pop and being a bit crap because the bottom end has done a lot of miles anyway and.
So IMO the difference in price over secondhand is negligible in this case, compared to the effort of installation and final result.
Result one - total costs including all the bits I want to do, new radiator etc - I'll probably have sunk about Â£800 into the car, but it'll have a new engine and clutch, new radiator, perfect coolant etc.
Result two - total costs around Â£550 including the new radiator etc, and I have a car with a probably 60,000 mile engine in it.
Result three - total costs around Â£400 I reckon, and I'll have a 127,000 something clutch and engine, and will have probably put a lot more work and faff into it as well.