I think a gun belt is critical, I have regular leather belts that used to be great that started to stretch after trying to tighten them up to work with a holster. I really like my Blackhawk nylon belt now, it keeps my nickers up and can adjust to my changing weight.
I am partial to my holsters (shocker!) but I think they carry well without shifting due to the wide platform. If you get a holster with good leather that's ready for sweat it it shouldn't be an issue. For example- the black leather I use is treated with a water repellent dye that makes it sweat tolerant. I don't wear undershirts under my IWB holsters and they (and my pistols) are fine.
That's my free, shameless plug. Take it for what it's worth

-Let me know if you want to give one of mine a whirl, if you don't like it I'll buy it back.