My advice in this case is to buy him a computer from Dell, and I'll tell you why, since this is coming from someone who builds their own machines, thinks everyone should learn to do it, etc.
Your dad is a businessman. He's going to need a computer based on its dependability and level of support, as well as to some extent, its features. While I'm not saying this can't be done with a built system, if this is your first build, you're going to be going through a learning experience of your own, which is best done with your own system that probably doesn't need to be a "When I need it, I need it now, I can't afford downtime" kind of system. Also, if you build a system, you are now the hardware and software support for that system. While I do a fair amount of support for my parents, those are personal systems. If it were me, I'd much rather that my dad had another option for tech. support to turn to, if I wasn't available for one reason or another (though I'd certainly be happy to help him). You can get a three-year warranty from Dell that will mean that if something goes bad, they send him the part and you put it in, or they send out a tech and it gets fixed, usually next-day. In business, this is important. Having built systems as well as ordered them and consulted for businesses, in your shoes I'd just go to Dell's website, customize a system to the way you think it should be, then take the printout and call a Dell small business rep, who will probably give you a better price than the website quoted for the same thing.
I'd recommend building your dad's home system...his work system, I'd leave to Dell.
P.S. If you build him a system, remember to buy licensed copies of Windows and other software he needs, and factor that in...Penalties are pretty harsh for not complying with software license when you're using it for business purposes.