MEKP (catalyst for Polyesters and Vinylesters) is hazardous, but with care can be used safely. It is routinely used commercially and is, the curing agent for Bondo.
No matter what you use, be smart and be safe.
I have helped a number of people with composites for rockets (I work with composites as the 'day job'), I suggest that they use epoxy resins for a number of reasons including;
More forgiving of use temperature
tougher - better mechanical properties
surface cure without additives
less shrinkage during cure
Polyesters have advantages too,
adjustable gel times (can get them so they don't run)
quick cure times
can use gel coats
works well with mat glass
I think the advantages of the epoxy offset the advantages of the polyester for making parts, polyesters may be better for mold making.
what ever you use, be smart, safety goggles (not glasses) are a must, have good ventilation, keep the area clean, have a fire extinguiser handy, wear a protective hand cream AND gloves (use two pair of gloves, that way if you need a clean hand, pull of one glove and you still have one on). After handling chemicals wash well with COLD water.