Desktop vs. Laptop will depend on the business they're thinking of starting. If it's one that won't require them to be on the road much, then go desktop -- you'll be able to get a lot more power for less $$$.
The brand name isn't a big deal -- I would recommend sticking to a major brand name like Dell or HP, for the support they offer. That is, unless they're computer savvy -- in which case, I would have them shop around and even configure one themselves. They might even want to look into leasing instead of buying -- it would be a monthly business expense, and they wouldn't have to worry about it going obsolete in two years.