For decaf I love a blend of 70% Colombian Decaf KVW with 30% Mexican Swiss Water Process. The Swiss water process gives a nice body but loses all brightness. The KVW process leaves the brightness
Slow roast to medium. Decaf roasts quick so you have to be careful as it goes dark quickly
I also enjoy mixing something bright like a good Kenya AA with a dry processed bean like Burundi that gives maximum body.
Just started blending for espresso a few months ago. So far the best results have been a very simple Brazil Fazenda Aurea and Costa Rica Finca La Ponderosa. Those are two really amazing coffees all by themselves so it's not much of a surprise. Right now I'm experimenting with those two plus a little Sumatra and Yemen. I have some really nice Guatemala in the cupboard that's my next adventure. Brazil, Guatemala and... I dunno.