I want to make a 2-part mould roughly covering 3.5sqmtrs but the overall cost is quite large.

Would it be possible to use a cheaper alternative to the Uni-mould Tooling resin (which appears to be the most expensive part) such as a cheap Polyester resin to bond all the layers of chopped strand mat? Or is there another cheaper alternative to the uni-mould system that would give adequate results?

I don't intend on pulling hundreds of releases, at most I would say 5 in total?

GP resin is fine mate, it is the cheaper alternative. Hell, even some big companies still build tools with polyester laminating resin. It just isn't quite as good, but you get what you pay for. You will still be able to produce a very useable mould.  

What about the gelcoat/coupling coat?