I would think that building high end custom machines would have to be secondary to the business.
The 'big box stores' and internet outlets have the low end machine market. [It is to expensive to
turn a profit on producing low end equipment when price is the sale. Out of the Box computers are cheap, cheap.]
Maybe an internet/gaming cafe with servers featuring high end gamers, but mostly internet gaming/surfing
stations could be an option.
The trick would be controlling the environment. Protecting your investment.
Today computer shops struggle. You don't need a computer to do the lions share of internet joy .. ie. cell phones, tablets.
Most 'enthusiasts' know the equipment and how to tweak it.
Not to discourage you. Many things have been accomplished by those that have been told you can't.
Think of ALL the factors ... first is location, location, location .. target your perceived customers.
Best of Luck .. but know that it will be uphill all the way. .c.