Jimmy I need to clarify. If you have an established relationship with a GC that you trust, by all means have them help prior to offer. The issue with investors, is that we tend to analysis many deals. If you have contractors visit every deal and give you bids and you get 1 out of 10 or 1 out of 20. They will not want to continue helping. You need to be comfortable with the offer you are putting together.
As far as hiring a GC to do a bid to cover everything. Yes you can. For me it really depends on the scope of work that needs to b completed. If you are doing a complete renovation that is extensive. It may be best for you to have one point of contact and one contractor that is responsible to you. The problem with this is if you need a new furnace and a roof, you don't necessarily want to pay a GC markup on the furnace that you could have done yourself.
I am currently adding an investor services side to my business that will address deal analysis, estimating and project management. We will be do these tasks for investors and teaching investors how to become proficient in these areas.