my son got a "Throne of their own" for his first birthday, I put batteries in it when he was two and let him sit on it. When i got serious about training as he neared his third birthday the batteries had worn out and i just never replaced them, He didn't seem to care much one way or the other, if anything the noise it made, caused him to jump up to try to look to see where the sound was coming from. so i personally don't think you need to bother much with any extras.
just wanted to add, that i never used the type of seat that went right on the adult toilet and we never had a problem transitioning to using the adult toilet when they were well trained. My kids are both on the petite side so it probably would have involved alot of messing around to get a stepstool have them crawl up on top etc and i didn't want to have anything hamper them from just dropping their drawers and going if the needed too. just my two cents on that.
My mom also bought us one that was a step stool, that was actually the one i prefered because i could put the "lid down" since I kept it in our kitchen, we lived in a house with only one bath that was upstairs, so for convenience sake, I just kept the potty in the kitchen where my son could get to easily and quickly and it just was more descret than the one that didn't have a lid.
Have you heard the tip of lining a potty with a plastic bag and keeping it in your car if you need it while traveling with a potty trainee??
Good Luck, I think 18 mons is the perfect time to start training, They learn quickly and are so eager to please you.