I was told ONLY invest in corporate guarantee properties, but I liked that DQ so I submitted a $500k offer, but didn't end up getting it. 

Like someone said earlier, the purchaser could have incorporated repairs into the purchase price.

At this stage from a boat slip rental perspective the cap rate is only about 5% but it could potentially be 12 lake front town home/ condo lots with two open water lots, it could be used for RV spots, or it could be a commercial bait shop/ lake front restaurant (currently there are no restrictions, property is in an un incorporated town).

While notes are different than buying the property, you do need to incorporate lessons from them in buying notes.

A close friend of mine that has been in real estate for decades took a look at my analysis in detail and after incorporating his suggestions (for example, he said the condo fee will include insurance for the common area and the building but the lender will require me to buy insurance for the rest...I hadn't factored this in) and your suggestions, my cash flow figure has swung from +200K to -227K.  

In your opinion what would be the most tax efficient way to start investing, for example would you recommend incorporating a ltd. company, if so under what jurisdiction?

My initial idea is to offer a bit more than I would be willing to pay - that would incorporate "interest" into the price. 

The solo 401k may be more advantageous if you plan to invest in real estate and incorporate debt financing (non-recourse loan).   

@Christopher Reynolds In general, there are many features that can be incorporated into a 401k plan, but are not offered by employers.

Has anyone used Guest Houses or incorporated them into their REI strategy?