The thing is, you don't need to be a modder to use mods if you so prefer and even unmodded, you can get a lot of mileage out of the game. I also play mostly SP and there are a few mods (as well as map tricks) to make the AI more challenging. You can always ask for advice on that and you will get it swiftly.
LDiCesare did a good job of listing the main points of improvement, which along with the bug fixes and new content make the game much better than Dominions 2.
Seriously. If money turns you off, 55$ is cheap for what you're going to get. I try to force myself to play other games, but can't. You're saving money because you won't feel the need to buy other games.