The last house (3unit) I purchased I went back asking for $8K for repairs (sewer stack replacement, electrical updates, boiler fixes, etc).

Sounds like there are repairs that need to be made to meet lending/fannie guidelines? 

A serious decline in that number would tell me there's trouble ahead.   

Overall it brought in a net annualized return of $4284 a month or $51,409 net annual (after management, vacancy, wear/tear repairs and cleaning fees).

Now the last piece for me before I feel confident making offers is getting a grasp on a potentials deal's rehab needs (the rehab part is my big blind spot as an investor).I've put together a list of about 10-12 contractors to initially contact in the area, but I feel a bit hazy on the bid process, and how often you can ask contractors to look at houses and give you an estimate on repairs.

After you have reached this number, you use CREDIT to pay for any repairs that come up and pay off that credit with your 40% surplus income each month.

Some single families in need of some repair, combined with the premium rents for SF's ( especially if you allow dogs) might be an interesting alternative. 

No Rehab/Repairs.  

This should include: Pictures of every square inch of the property (I recommend uploading them to dropbox and sharing the link) you should have 50-70 pics.Accurate and updated comps of the property and an ARV (after repair value).

Vacancy, repairs, cap ex, and management.