Add-a-leafs will stiffen the ride, especially one that is to give you a 3.5" lift. If you had the money, I'd do the whole spring pack because add-a-leafs are kind of a "cuttin' corners" route. I've done both. The OEM leaf springs aren't going to be as good as the RE spring pack so all you're doing is adding a decent aftermarket component to a relatively mediocre OEM spring pack. And, under enough pressure the AAL can break the OEM leaf. In cases like this when you're first building up your Jeep, you tend to go a bit cheaper at the beginning and end up replacing most of the parts with what you should have bought in the first place (been there, done that as most of us will attest). So if money isn't the issue, do it right the first time. Places like Wyotech, if you're familiar with who they are (look them up if not) do not allow stuff like AAL's and blocks because of their unreliability and to a large degree, the safety.

Tires... I dunno, I've always been partial to BFGs, they're one of the best for a reason. I've heard that some tires wear pretty quickly, not sure which ones, but BFGs seem to be one that isn't too soft and isn't too hard, so you can get some mileage out of them, especially if you rotate your tires regularly. You might consider going to Discount Tire and getting their Unique brand black four-wheeling rims at about $30-40 each ($28 when I bought mine) as opposed to the $75 for ProComp. They look pretty much the same and if you're only doing some average wheeling, then there's really no reason to buy something twice as much. They are both steel, I can't imagine if there is much difference between the two, but you'll never notice I bet. 

You'll love the Cherokee and it'll last a long time, so do the best you can afford or you'll be replacing stuff as you get into it more and realized where you skimped. Except the rims, save your money there and get the Discount ones.

HTH,

jon