I'm evaluating to buy Dom3

[Sombre]

One thing to remember is that there are a lot of mods already for Doms3 and more around the corner. These add new spells, units, nations etc to the already massive lineup.

From what I understand MP dominions is not something that you're likely to get tired of and with all the original content and mods it's unlikely that the game will get stale for you any time. In my opinion if you're getting it purely for SP then you have to consider how much value you're going to be able to get out of it. You can certainly create a more challenging game by giving the AI more resources, having maps where they're allied, giving them overpowered mod nations etc, but you're still not going to have the fun of a human player using a trick like stealthy troops or coordinated overland spells on you.

This is true of many games, although Dom3 is missing a big SP feature in that there's no 'campaign' structure to play through.

It's an expensive game, so read the reviews and play the demo first. But you knew that.