Mike Lehman gave you a pretty good rundown on the RTR side of HOn3. I started in HOn3 a little over a year ago. I’m on a limited budget, putting the detailed brass out of reach. Also, at the time, the Blackstone and MMI wren’t very close to production, and the K-27 is a bigger and more modern locomotive than I wanted anyway.
In my locomotive budget (note Roundhouse, Ken Kidder, FED are out of production but still available):
Grandt Line box cab and 25T diesels (kits) ($50?): 4 wheel diesels “critters”. Some have the bodies recast in brass for extra weight and pulling power. The drive needs to be worked carefully to get all 4 wheels to provide tractive effort.
Roundhouse Shay (RTR and kit - RTR about $125-$150, kit about $70, easily found on eBay). Often does not run well out of the box. Needs tinkering, NWSL parts help, can be made to run very well. Nicely detailed, and easy to modify/backdate. Some consider the frame too large for HOn3. I have standard gauge version.
Roundhouse 2-8-0 (both inside and outside frame versions made, almost all production was kit, available eBay and some dealers for $80 or less). A good kit to learn how to build kit locomotives. I have one of these. Matches a Baldwin C-25 reasonably in many dimensions.
Keystone Shay (non-motorized kit) Needs NWSL motorizing kit, which is currently out of production but available. Makes into jewel of loco but kit is at a craftsman level. I have one of these also, may do a second.
Ken Kidder imported brass models of Porter 0-4-0T, 0-4-0, 2-6-0 in the '60s in both kit and RTR. Very sparse detail. Available on eBay from $50 to $125, depending on model and condition. I have the 0-4-0T.
FED imported both 4-4-0 and 2-6-0 as low-end brass in late