Need New Bike For Electric Conversion

Hello to everyone!

I want to re-build my electric bike for the summer! I currently have all of my parts on a Mongoose Blackcomb that I purchased used from a friend for $50. It served its purpose, but after 1000 miles at 40mph it is starting to show its price.

I need a new bike, and my budget is,,, well I spent all of my cash on the electronics so I only have $400-$500 left for a bike. I will turn 18 this summer and I am a Junior in HS. I am in too many sports to get a job until the summer. It has stopped snowing much and the lawns won't be ready to mow for a few months, so money is really tight ATM. The bike needs:

-Steel rear dropouts (for the motor)
-Full suspension
-F/R disc brakes (can always upgrade the front ones)
-Place for batteries (but I can get creative if needed)
-Room for 2.4"+ tires

I only weigh 150lbs but I let people ride it that weigh more so weight doesn't really matter. Weight is further irrelevant because I will be adding 15lbs of batteries and a 14lb motor among other parts. However, the rear suspension needs to be up to the task of containing the 14lb rear wheel!

I ride on pavement, gravel roads, and bomb across the occasional field or two. I need the full suspension for the long rides on gravel roads! I won't do any jumping.

Now for the new bike, I was considering a FS Dawes - https://www.bikesdirect.com/products/dawes/rh_25.htm

I would also buy used, but I am a little weary about shipping. Pinkbike also... the prices aren't always what they appear on the listing and I don't want to sound like an idiot lowballing people. I don't want to spend much over $500 after shipping and it shipping is a gray area to me on pinkbike.

Any suggestions?

(direct links) Here is the bike in its current state. I have the ski on it now and when the conditions are right it's fun as hell! And here's a link to some videos.