I've had dual sported KDX200's and KDX250s (two strokes) and an XR250 (four stroke, bored to 277cc).
I would carry several small tupperware bottles (about 9 ounces) of oil in my fanny pack. Add about 3 ounces per gallon. Just put a little oil in the tank, add some gas, add some more oil, etc... The swirling of the gas entering the tank always seemed to mix the oil just fine.
With a bigger than stock countershaft sprocket, a two stroke 250 should be able to top out at over 80 mph. However, it is not comfortable to cruise at over 60 mph on most bikes, as you are turning a lot of rpm, even with taller gearing. If you gear the bike too tall, you will struggle on tight trails, too.
Basically, most four strokes rev higher and produce usable power over a wider rpm range, which is useful for road riding.
Several issues with 2 strokes you might not have thought about: Rolling off the throttle after a sustained high speed run is BAD unless you pull in the clutch. Hot engine + high revs with no cooling fuel and lubricating oil can result in catostrophic failure. IOTW, don't use engine braking to slow you down after you have flogged the bike, and you will be fine. When they used to make two stroke street bikes, they did not use pre-mix but had oil injection driven off of an rpm sensitive pump to prevent this problem.
Secondly, on my KDX250, highway cruising was irritating because the bike did not like to hold a constant speed. Basically, if you held the throttle still a crusing speed, the powervalves would start to open and the bike would speed up, so you would have to back off the throttle. Then it would slow down. Then you would open the throttle a little, hold it steady, and the bike would keep keep speeding up. Gets annoying after awhile.
Also, to add a larger countershaft sprocket to permit lower revs on the freeway, you may have to eliminate the guard that protects the case in the event the chain derails.
I would go about 1,000 miles between rings and 2,000 miles on a piston. With a piston, a top end is usually about $120 US. Just rings, gaskets and wrist pin bearing maybe half that.
On my KDX250, I would go between 90 and 100 miles on a 3.6 gallon tank. My XR250 would go 85 miles on about 2.5 gallons. With most small bore bikes, you will actually get worse mileage running on the highway at 60+ mph than if you are cruising country roads at 30 mph. For most road rides, you will get slightly better fuel mileage than you do when trail riding.