No problem using a slide stop thumb rest. It is easy to try without drilling your frame. Drilled frames seem to lower the value if you ever decide to sell.
I disagree about about buying a 38 SC over 9MM gun. 9MM bras is ubiquitous, easy to find, very inexpensive and most importantly safe. I will send you by PM a builder many in the midwest have used and all operate reliably.
The overwhelming majority of open shooters use STI framed guns based on the 2011 platform. I think it would be a mistake to stray to far from this platform. There are several custom builders that will build a polymer gripped custom gun within your budget.
Bullet - Precision Delta, Montana Gold or Zero 124 JHP
Primer- A any small pistol or Federal Small Rifle
Powder- Silhouette or Aotocomp
OAL- 1.165
7 Grains of Silhouette = 171 Power Factor.