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?

A rapid tooling system like Unimould will produce a much better mould than regular GP laminating resins. The main advantage is time, it is very quick and easy to produce a tool because you can layup the bulk reinforcement in one go. You can do a complete mould lamination in a few hours and have a ready-to-use mould 24 hours later. This is more important for professionals than hobbiests, but still a great advantage to have.