We have an architect who's helped us draw up plans for converting the first floor into a commercial space and the 2nd/3rd floors into 5 apartments/condos all about 2,000 soft in size.  

An architect will have a ballpark but +/- 15% accurate.

An architect developer in San Diego named Jonathan Segal did something similar in addition to his main development business.

I think it depends how confident you are the property will close and how much the architect/drafter is charging and what your holding costs are.

Will I need an architect to do drawings?

I do have an architect currently developing some 3D blueprints for me. 

Hire an architect to create your full set of CD's (Construction Drawings), with the component plan sections as follows: Architectural, Mechanical, Electrical, Plumbing, Structural, and (sometimes) Civil Engineering. 2.

Some other costs to consider is Architect plan costs, Financing holding costs.  

You need a capable architect to really tell you the full impact that this would have.

You also have to have an architect available to start work on plans as soon as you get a contract or close on a house--that typically takes months.