It depends on if diesel or gas. I think in this situation and guessing that you may not have a mechanical aptitude, tools etc. I think anything sub 100k is great. My ratio for acquiring vehicles used to be Under 10 years old, under 100k miles & under $1,000 now that recipe is over 20 years old & needs an update for inflation but the other two parts still makes sense. Th edollar amount really doesnt matter & nothing is exact but I am a cheap bastard & have spend decades searching out the best deals of r the buck. The issue with all vehicles if time, neglect & just normal wear & tear. We are both located in northern climates with salt use so prevalent rust so there is always more rot to consider. I see some rust on the door so must assume the undercarriage looks rusty. If the frame is solid then its usually only brake lines, fuel lines & brake pads/rotors. the pads & rotors are easy enough if need be when need be. Any backyard mechanic can do them The lines can also be Jerry rigged without a flaring tool tubing bender by getting sections with couplers & tubing.
Of course there are major mechanical systems that could be unsuitable for cross country traveling in any 1981 vehicle and even a 2011 vehicle. I gave those as exqamples & they all have their pros & cons. The unassuming white bubble van is a good stealth camping conversion as its without windows & looks like a work van but can easilly sleep in any city street parking spot. The 2 camper sare a bit rough and with some mechanical/carpentry skills someone could convert a party bus, school bus or more modern e450 for about the same cost that will actually have more living space but be more home made. $3,500 is about $1,500-$2k too much for that 1981 camper so whatever your true budget is; you wouldnt want to blow most of it on acquisition. Remodeling, registering, upgrading mechanical systems plus fuel gets pricey. Its all stuff to consider. the time to buy converted skoolies, motorhomes & vans for traveling is always in the winter/off season.
I got nothing against bikes & bike touring but here is something to consider. You can always bring a touring bike along with a vehicle. If the vehicle dies & is too expensive to resurect then you can easilly acquire panniers & start pedaling. Regardless, I wouldnt put all your eggs in 1 basket by spending $3,500 on that RV. If you can grab a solid mechanical vehicle for sub $2k then spend some money to pimp it out, you may have a more dependable ride. even less you might be better off. but I would rather see you in something newer for that price. Any frame damage would be a definite eliminator for any vehicle, IMO.. A SBC 350 motor is really common motor but it could be really tired. The motor could also be strong but one must consider all of the suspension, drivetrain & other mechanical stuff before any of the camper type stuff is determined if in working order. I do like the vintage Class c and regardless of Van, Short bus or RV; you have the ability to recruit others that can help you repair it and or take on riders or travel partners to help with fuel & company.