regular polyester resin can certainly work! uni-mould or epoxy would be better, but like you mention the price of polyester is way more attractive if your on a budget. 
The most important thing you'll have to take care of if making sure you don't "overheat" your mould. By this I mean, making sure you don't add to much resin a fibers in one time because this will lead to a higher exothermal reaction. 
How can you solve this:

1. only add 2 layers of +/- 200g/sqm at the time and reduce the amount of mekp to a level you feel good at (manufacturers will recommend something inbetween 1-3% MEKP) 
2. be extremely careful with your corners and edges not to add to much fibers or resin in one go (to much will lead to higher exothermal reactions, causing more risks for shrink and warping) 
3. apply new 2 layers of 200gsm after each layer went through an exothermal reaction (can take a couple of minutes/hours/days depending on the amount of MEKP you've use)
4. after every layer has cured (make sure it didn't cure to much) clean it with acetone (some polyester resins will have a wax-additive for better curing to open air) and scratch your surface with a 60grit sandpaper and clean with acetone before adding the new layers.
5. Once you have all the layers on wait as long as you can before demoulding (3 days would be good at 25°C) and do a postcure in an oven at 45°C if that's an option. 

If you repeat this progress of adding 2 new layers/clean/scratch for a couple of times you will be able to build up something like 6 layers of 200g in 3 times.

This is how I used to make moulds before I got to know the Uni-Mould system; which is TIME-wise a CHEAPER option as you can do the entire build up in 1 go with minimal risk of warp or shrink and demould after around two days if you have time.

Note: This is just to give you an idea on how the system works, various elements can be adjusted to something you feel good with like adjusting the amount of MEKP, numbers of layers of fiberglass you do each time, or gr/sqm of CSM you use.