Former manufacturing engineer from IAG here...
you're plenty fine with stock sleeves up over 1000whp, the sleeves aren't the thing that fails. If you have a cast iron sleeve fail, trust me, you aren't just replacing one sleeve, you're throwing out the whole thing and starting over again. The cost of a comparable closed deck to a sleeved block is what you need to look at, they both need to be treated as a consumable, because they are... If the used close deck block you're looking at is an IAG block, then if would not be successful to install sleeves into it at that point.
The cost of the sleeves themselves, plus the price of the machine shop to do it correctly, plus the long lead time typical of the places that can do it correctly are things to consider.
If you have any questions, i'll try to keep an eye on here more often!
good luck with you're build