1st timer need advice on buying used

You probably won't be going for the whole 4 days pulling 8500 lbs.. Days three and four will be spent at a garage. Forget about the GVW and GCWR of your vehicle. The important part is payload. If you have 777 lbs. of available payload and assume a tongue weight of 11-15% of trailer weight, you should be looking for trailers, fully loaded, in the 5500 lb range.