Well, as a player who often play these guys I guess I could give my opinion here.
All in all, I like the ideas, but some things could be better to make them stronger, but I guess its not about making them excessively strong, just giving them more flavour.
* -10 idea cost: Probably the best idea for its spot, it will help greatly to knock out that all important Exploration idea-group early on and afterwards saving points on ideas, thus making it somewhat easier to catch up with the europeans.
* 20% extra forcelimits: Very good in later game, but not that usefull at this spot since you probably conquered most of your rivals by now and more men doesn't really help that much against the Europeans, its more usefull at a later stage, at which point you can actually become quite a beast with this.
* +1 Prestige: Kinda meh, but you know, it adds flavour.
* +1 Colonist: Very good, but a bit late... would be far more usefull if it swapped with the Prestige buff.
* +10% tax mod: Probably better if swapped with the forcelimit mod, but not complaining
* -20% build cost: very usefull in the "catching up" phase where you need to rush up your infrastructure after westernising to be able to withstand the europeans.
* +10% production: This allso equals better colonist chance, but by now colonising might not be that important anymore, but it's okay.
* -10% land attrition: Personally I hardly (if ever) attack stuff, so this is a bit underwhelming, but it canbe quite usefull I guess.
* +1 leader: Kind of okay
No diplomatic abilities? Most Inca conquests involved no battles, just a show of force and a few bribes. Meh, these seem a bit cliche but they'll do.
I think that would be far more appropriate for it's precursor Cuzco/Andean ideas than for Incan ideas (you now, they could be planned as well)