If the community checks out, my girlfriend and I will be relocating to Norfolk in pursuit of investing, supplemented by our full time jobs. 

I would like to supplement my retirement with rental properties in 11 years when I am 55.

The good investor agent will provide you w/ access resources to supplement whatever resources you may have.    

I was outbid on a town home near the west Seattle super supplements a few weeks ago and we are in the process of submitting an offer for one in the south Delridge area.

However, they are re-doing the exterior and have not collected enough or banked enough HOA fees to cover this exterior cap expense so there is a supplemental charge to each owner that was approved by the HOA. 

Does it make more sense to buy a condo that I can pay off in ten years or buy a starter home for about double the price and have a rental unit to supplement the mortgage?

Acquisition fees and asset mgt fees keep the engine running from the beginning thru the hold period, the real money is made by the GP at sale or some portion at refinance / supplemental loan when property renovations are complete / value optimized. 

However, I want to keep that going and at the same time establish passive income through multiple streams to supplement my retirement.

Should we put down a 20% deposit for our first home, or should we put down a smaller deposit and use the leftover cash to pay for an ADU that will generate supplemental income?

No I wouldnt I dont over pay knowingly ever but for someone who wants to supplement their retirement with out having to deal with houses all over the place over paying for the house next door might just work for them.