I’m now running a SRAM brake lever on the left, and Shimano 105 on the right. Originaly had a SRAM Rival STI shifter but ditched that after it broke.
The alloy SRAM brake levers are good if you need a new brake lever anyway – light and simple. Don’t bother with the carbon as the weight saving is very minimal. But yes, to save cash you could just use the original LH lever (gut it if you want).
If you want a wider cassette (I run 11-32 normally) you can get a 105 mech with a slightly larger range to cope with that. 5701-A I think.
Personally I would avoid MTB mechs as you would need an in line cable adjuster to adjust for cable slack (whereas road mechs have those built in obviously).