Gavin, you really need to take your time. For the kind of money you want to spend there are LOTS of RVs out there. The trouble is how were those RVs used and abused and cared for. Find a few you really like. Find an RV technician to check them out for you. I don't care how knowledgeable you are, if you're not knowledgeable about RV systems your knowledge is useless. All else being equal, look very carefully at storage and floor plan & remember you'll have to live with it. Something that wouldn't bother you for a weekend might drive you crazy in a permanent situation. Unless the seller can PROVE to you that the tires are less than 5 years old, you WILL need to buy tires (learn to read and look for the date codes ON the tires & take NO one's word for it.). You may need to have a professional take care of some of the systems if it has not been used often. Look MOST carefully at the roof and side walls. If the roof leaks or delamination has begun you may be opening a huge can of worms.
In this economy it's very likely you can find a great, not too old RV for your money & find it in great condition too. But without a pro to tell you that, be very careful. Also, high mileage is NOT a bad thing. High mileage means it's been worked and an RV that has not been worked is more likely to have problems that have been neglected due to lack of use. High mileage on a diesel means it's just been "broken in". Diesels can easily carry millions of miles. Gas, not so much, but a gasser with high mileage has probably been taken care of & as long as you are careful about maintenance too, no problem.
MPG? Pooh. You are talking about driving a house here. If you manage to get 8 mpg or better on an RV built before 2008 it will be a miracle. Figure the cost of fuel into your expenses and let it go. It won't get better. There are so many other expenses you will be giving up by living in an RV it shouldn't matter anyway.
There are numerous options for wifi. For me, the best one has been Millenicom, offering 20gb per month for 70$. They use Verizon's towers and support is excellent. They have an unlimited hot spot plan also, which I had originally, but after you use up so many gb you're going to get slowed down anyway & the additional cost was not worth it to me. If I'm gonna be slowed down to a crawl anyway, I might as well switch to campground wifi if I go over my allowance (which I never have). If you plan on streaming live content however, all bets are off. You will never have enough bandwidth unless you have a satellite provider, which is expensive.
Best wishes.....