there seems to be numerous pre-purchase analysis software programs for creating a pro-forma for multifamily Acquisitions, available in the market.

Do some research into other rents in your area and come up with an estimated "market rate" for the rent (which would be your "pro forma" rate), and base the price on it.

If you don't have a lot to loose, in way of $$$ and assets, than just keep it simple and form a LLC in your state where your doing operations.

My rough estimate is 50k-60k for rehab to get it to pro forma level. 

I have some friends and business partners who may be willing to form a partnership. 

Should I form a legal entity before buying the property of go with my name for first few property and later change to llc?

At least for the prop-forma stage.

Austin Peck we use our pro forma in excel to play around with numbers that both make sense for us and the seller.

It only complicates your business and makes it more difficult to operate.If you actually have a good accountant I would be very surprised if they recommend you form a corp.

Plus, taxes seem to range a bit.I've heard @Brandon Turner talk about including property management with your analysis from the start, so you can easily start using a property management company and not impact your original pro forma estimates.