Do you expect to immediately make a living building professional computers?
Or work up to it while working part or full time at your regular job?
What income do you need/require from the business to pay bills and reasonably (relative) maintain your life style?
Use your current income. If you make $1,000 USD per month then the "profit on one computer would need to be $1,000. If two computers per month, then you would need $500 per computer.
But the customer's price must include the parts, etc.. And you must allow for other expenses: consumables, tools, shipping, travel, etc.
Does not matter per se if you build and/or provide support services.
The concept is known as a "Breakeven Analysis". Crunch some numbers. Go low with income, high with expenses. Best case, expected case, worse case, situations.
Plan on 80 hour weeks. An "easy" 1 hour repair response can quickly become an all night nightmare.
Charging by the hour: Take your current 40 hour per week salary and double or triple. You can charge by service (e.g., adding memory) - some flat amount if you wish. Again, a 1 hour job could end up taking much, much longer.
After Breakeven Analysis you must look at cash flow. Your suppliers will want their money up front. Your customers will not want to pay you until the work is satisfactorily done. And if the customer is a bigger business they are all too likely to drag out and hold of paying you. And your rent is due....
Do some research in your area. Look for organizations that are there to help (hopefully for free) small business start-ups or struggling existing businesses.
Your idea is not at all unusual and many people try starting all kinds of small businesses every day.
I am not being negative although it may seem otherwise. All in favor of your hopes and dreams. But go into them realistically and understand the risks.
Almost every small computer shop in our area is now gone. The ones that are left - unfortunately not much hope for them. I have visited a couple as I do prefer to support local small businesses. But what they offered was much less at significantly more cost.