Ford, Chevy or Dodge?
If this is your first gasser, are there any folks you fly with or at your club who have gassers and might give you a bit of help, or who's airplanes you might see fly?
While there are a number of engine brand choices in that range, around here the two predominant ones are DA and ZDZ. I faced the same question when I started with gassers and went with the ZDZ mostly due to the fact that I'd seen them perform and talked with people around here who had them. Part of that decision was due to the fact that the DA's were back ordered when I wanted an engine as well. I now have several ZDZ's and have been quite happy with them.
Gassers are not a big mystery, but there is a bit of a learning curve and local help to get you started will make the transition easier and more fun IMHO...