think tank or lowepro
if you want to hold out for ebay deals, you can get them for a great price. The lowepro has a flaw with heavier setups, and your stuff can end up on the ground! There's a fix for it by using a metal buckle. The think tank comes in a few varieties. You can adapt think tank and lowepro bags to each belt system without too much difficulty, I happen to own both.
If I were still shooting DSLR's, I'de lean lowepro since the belt is super heavy duty (along with proper belt buckle mod). Think tank is my preferred belt for a smaller system.
if you REALLY want to get inexpensive, you can also use a toolbelt from home depot. Leather with metal buckle may run around $15-20, canvas style around $7. Both will easily carry a mirrorless system or just lenses. I've got a few toolbelts and they work fine. Think tank is more comfortable due to the padding, but you can definitely save money with a toolbelt. As for attachments, I love the think tank skin body bag. Put the divider, and you can fit two DSLR lenses in there and the front for small accessories. Disclaimer, the bag has no padding.... not a big deal IMO, since it stays close to your body. If you want a padded bag, lowepro makes a similar sized bag, almost like a utility bag. Not as good as think tank's but cheaper and padded.
Sony A7siii/A7iv/ZV-1 - FE 24/1.4 - SY 24/2.8 - FE 35/2.8 - FE 50/1.8 - FE 85/1.8 - F 600/5.6 - CZ 100-300 - Tamron 17-28/2.8 - 28-75/2.8 - 28-200 RXD
Panasonic GH6 - Laowa 7.5/2 - PL 15/1.7 - P 42.5/1.8 - OM 75/1.8 - PL 10-25/1.7 - P 12-32 - P 14-140