Having wrestled with seats over a few different E-Series the guys in your link are simple GREAT!
Their approach is sound, their cost for the foam cushions are very much in line with what the dealer offers. I recently had an '05 drivers stock bottom seat repaired as the outer fabric was trashed as they point out, a bit of foam added between the beat up original factory foam and new upholstery---cost me $110. Had I known about these guys I'd have used their products.
The video found in the link is great as well---as with all things E-Series seats remove them first.
They don't say it takes a T-55 Torx PLUS bit and an 18mm wrench or socket to remove the seat base from the body. After that point what they share is quite generous IMHO---they demystify how the upholstery is removed and reinstalled.
I myself LOVE leather or at least the idea of its sensual feel and smell however the reality of that material falls short of it being long lived and not really well suited for any sort of SMB or adventure van---IMHO mind you. I'd opt for new bottom foam, a new cover if the exiting piece is really in need of replacing then top that off with a nice Cordura form-fitting seat cover. I've purchased and would highly recommend a LA outfit called Auto Decor--I think several other members here have also purchased from them.
I'd also add their Durofoam cushions seem to be a big improvement in the factory pieces. If they're half as durable as claimed they'll be well worth the cost they ask.
Overall I'd buy from The Seat Shop because I like their approach. Again their costs seem fair considering they have a wonderful understanding of the E-Series seats.
Please let us know what you eventually decide on doing.