hi hi, welcome welcome,
Anyway, the models (excluding other models eg. K200, etc):-
1. Holga 120S: Basic standard Holga, plastic lens, no features
2. Holga 120SF: Same as S, but with a built in flash
3. Holga 120N: Bulb mode, tripod mount, hot shoe
4. Holga 120FN: Bulb mode, tripod mount, built in flash
5. Holga 120CFN: Bulb mode, tripod mount, built in colorflash
6. Woca/Holga 120G: Like the 120S, but with a glass lens
7. Woca/Holga 120GF: Like the 120SF, but with a glass lens
8. Holga 120GFN: Like the 120FN, but with a glass lens
9. Holga 120GCFN: Like the 120CFN, but with a glass lens
'S' or 'N' - Obviously, get 'N'.
Glass or Plastic - well, depends on u. I've read people saying glass is sharper. though many still prefer the plastic lens, cos, afterall, it's a toy camera.
Flash? - it's good to have flash, especially for the holga. do note though, if there's a built-in flash, there won't be a hotshoe for a external flash. If u're going for flash, get the 'CFN'. 
so wat's ur budget like? 