I wear my MR Helmet with the neck piece under/inside the chest armor. I would add some moleskin or other type of soft material on the underside of the armor to limit wear on the helmet. In regards to the helmet that comes with the costume, though it is claimed to be "from the actual molds," it is more likely to be from some early molds of an ESB helmet. So, if you are going for authentic ROTS, you're in for a lot of mods, if you do not want to use the MR helmet. Some other suggestions:
1. Replace the suit w/ a leather one piece - makes a HUGE improvement
2. Replace gloves w/ leather - same improvement as above
3. Add an automotive quality paint job to the shin armor and chest/shoulder armor, along with a high gloss coat. Unless you have the funds to replace these with fiberglass, a quality paint job can do wonders for these lightweight, easy-to-troop in pieces.
4. Keep the cod piece as-is.
5. Replace belt boxes and buckle w/authentic sized - I have Portumac, though I know some would prefer others.
6. Clean/scrape out the red toggle switch on the chest box and add a red light w/ some diffusing paper.
7. Replace the rods on the chest box with metal rods.
These are only suggestions, IMHO, to get you by for a while. Other people can get you a lot farther. Good luck!
Jason