I have a Lavacore farmer john. They call it a sleeveless full suit. I have worn it (with Lavacore socks and a regular neoprene skirted hood) under a 7mm suit in 52 degree water and (just the farmer john) under a 3/2 suit in 72 degree water and been very comfortable both ways. I was almost too warm in the 72 degree water, actually.
I think having a good undergarment is a good way to go. I have that, a 3/2 suit and a 7mm suit and I can wear the suits alone or combine as needed to be comfortable in pretty much any conditions up to the point of needing a dry suit. I have less money in all that than if I'd bought a 5mm suit instead, and it lets me be comfortable in colder water than I could be with just a 7mm suit alone.
If you want a good deal on Lavacore stuff, my local shop has stopped carrying Lavacore, so they are blowing out their remaining stock for cheap. As a comparison, my farmer john would have been $180 from LeisurePro and it was $105 from my local shop.
Their website is
Coral Edge Adventures. Ignore the prices for LC stuff on their website. Those prices are "regular" price. Call the shop to get the sale price on anything you might want. I'm pretty sure they'll have no problem taking a phone order and then shipping it to you.