He took one look at the place and advised us that he was willing to give my girlfriend €10,000 for it and guaranteed with great certainty that she would never see more for the place.This devastated my girlfriend.

To prevent this from happening, find a company that does both restoration & renovation, and have them guarantee their work.) - Look for companies online based on rating system, so you can see other people's reviews. - Also, it's good to see how responsive they are to your calls/texts.

I think my current agent quoted $800/year, and I'm guessing I'll have to guarantee around half of that (this flip will be done - and hopefully sold - in 3-5 months).

Otherwise your guaranteeing a clear title beyond the time you owned it.

I could either go to school and take over his business and guarantee my self a 6 figure salary until it’s eventually mine and use the money I make early on to invest into real estate and other areas to create passive income and eventually just own and run the business not work there.

It can be pricey, but I believe they will guarantee the foundation won't move.Another option is to dig holes outside the house, and bury anchors.

Most of my rental buyers prefer Section 8 for the guaranteed government paycheck. 

They ended up asking for 25% down since it was my first one and had to put my personal residence up as a guarantee but they approved me.

I guarantee you there is a deal somewhere around you.

It will be more difficult to qualify the property for this, but you are all but guaranteed to get the long term financing this way.