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?

As you can correct the inaccuracies during the final bonding process this might work well with regular resin. As long as you are building grp hulls, you might just use regular Gelcoat for your molds as well. When doing crp you should use a polyester based gel coat like the stuff from easycomposites to avoid the epoxy/polyester problem. And you should try grp first to check if everything fits nicely