Stirlings are neat engines, both to build and to run, in comparison to steam/comp. air or IC. They don't emit anything except some fuel combustion products (H2O and CO2, if the fuel is alco or butane/propane). No half-burnt gases, oil, vapour, or any mixture of those. They're also very quiet, and run practically "forever" with very little attention.
As to what type of a Stirling, I prefer the gamma. Alpha Stirlings are not easy to get to run at all, and need very exacting machining. And a BIG temperature differential. Beta Stirlings, with coaxial pistons, are compact, but they don't have much of moving parts, and most of them are not visible (same as the alpha). Gamma Stirlings can have their piston rods, connecting arms, crank, etc easily visible, and I like my engines that way
(Engines such as a Rider-Ericsson or Robinson)
Low Temp Differential Stirlings are usually gamma type. All model Stirlings must be very free running, and especially the LTD, so that will be the all-important thing. And of course air tight. Even if you are not trying to break records on how low a temp difference it will run, you should all the time aim for the least possible friction.
Jan Ridders's plans have been built in great numbers all around the world, and if there's any model in his "catalog" that you take a fancy on, they can be recommended without ifs and buts. And there are lots of other plans available. I wouldn't try a very tiny engine as a first build. Bigger are easier to get running. And definitely not a set of castings. If you need to make a part many times to get it right (I at least often need to...), bar stock is FAR easier to replace than a botched casting.