Here's a quick answer, based on opinion.. I'm sure you will see a few others.
1. At your price-point, there is limited choices that are well-engineered. I think you should look at FFR, B&B and A&C. All are well-engineered and very buildable. However, none of them are the "put Tab A into Slot C" kit...all will require some fabrication skills.
2. It will be hard to stay under $20k, even with the entry-level kits...there are a few all-Donor FFRs that have been assembled for less than 20K, but the trend lately is to go new parts, which will bust your budget.
3. Sell for more? Maybe, but unlikely, especially with a budget build.