This is exactly why I buy Custom Built. Some things to remember.I know you have your heart set on 'custom built' which really doesn't mean a whole lot these days. I'm going to make an argument for an off the shelve system though. Now a days very capable/powerful laptops are available at your local Wally world for $5-600, maybe $750-$800 if you're looking for an i7 processor. At that price they're replaceable within in a 2-3 year cycle. 3 years from now you'll have much more powerful computers at your disposal in that same price range. It doesn't make a whole lot of sense to plan on a consumer grade system lasting any longer.
For me, I take my budget and buy the most bang for my buck and plan to replace within 3 years.
I will be running the same three programs so nothing will change.
My last custom built cost me $1,500. Coming to think of it, I think it was actually $1,200.00. It lasted me and is still working 6+ years.
I am buying more than is needed now, so by the time it gets obsolete for me, it would have paid for itself many times over.