Your approach seems fair-minded and sound for getting familiar with the work. You aren't really encroaching on those people's time, taking them away from pay-time activities.
Asking three contractors (or even one) to give you free estimates as some suggest isn't going to work very long. Contractors don't get paid for quoting. It takes them time and money to quote. You may get quotes out of them once or twice, but the third time, they will stop taking your calls.
Moreover, asking them to quote with the promise of future business is disingenuous, and unless they fell off the turnip truck yesterday, have heard before. They aren't going to greet that one with enthusiasm. Not only that, you sound like a real amateur even saying it and erode your credibility. Furthermore, if you think about it, you are stealing from them. Time they spend doing free quoting of a job is stealing time from them that they could use doing a job and making money.
However, you really don't need to take up a contractor's time at all to get a rehab estimate. Figuring rehab costs is something you can do on your own and is very formulamatic. Your REIA should have spreadsheets available for the asking. You pick the work that needs to be done on the spreadsheet, plug in the square footage of the area that needs the work, and chug out a number. If you know how to use a tape to measure a building and a room and how to put numbers in a spreadsheet, you can do this.
If for some strange reason your REIA doesn't have such a sheet, there are plenty available elsewhere. I haven't looked, but I bet there is one available here on BP.
Finally, the AIA (American Institute of Architects) publishes guides you can purchase that give you all kinds of figures for calculating costs.
It all boils down to a cost per square foot. Costs per square foot are known. You know the square footage; you can calculate the rehab costs yourself pretty reliably and pdq.
Alternatively, you can do what I did. Find a contractor who will generate a quote for you for a fee. Mine does it for $75 a property. It depends on how you value your time. Totally up all the time for the steps involved, it takes me a couple of hours. I consider my time worth more than $37.50 an hour, but everyone is different.