You really need to focus all your energy on buying the right house for the right price and producing a good product.Maybe you already have a great agent, inspector, GC, plumber, electrician, designer, etc.  

Question: Should I hire a home inspector or should I just hire a plumber, roofer, and electrician separately to do their own tests and inspections?

And this goes for any plumber, electrician, property manager, ANYTHING. 

Bad switch or outlet, call your electrician.

If you can have the price of hiring a licensed electrician correct the wiring taken off of the purchase then yes, and maybe some plumbing repairs the current owner should have corrected as well?

Find a plumber to do the plumbing work, or an electrician to do the electrical work and so on.  

We need a plumber and electrician to come in to start and finish rough work.

Calling electrical pros ----I am in a situation where power was out and an electrician tied the ground and neutral behind a GFCI and put power back on, but I am very worried that it is a hazard...

It may be possible to splice the old circuits in new junction boxes, but that may cause code issues.Best case scenario, $1,500 could cover it, but I think it could be more than that if there is substantial rewiring needed.It's probably best to talk to a local electrician to get a professional opinion.

Let's say you find an electrician who is very good and what he does.