Now, regarding a 200k mile truck, I personally see no issue with it, if everything is running & functioning well.
My current tow rig is a 2008 Ford Expedition. Has 223k? on the clock and still runs & drives well. No reason to replace it honestly, unless I wanted to start towing twice the weight, and/or use a snow plow.
At 200k + miles, the biggest "risk" with just about any modern pickup truck, is going to be the transmission. If the transmission is healthy, and everything else runs well, then why worry?
Often times though, if you feel/need something drastically different, you're usually money ahead to simply get the better vehicle, rather than "build"/modify your current truck to meet some unrealistic needs.
We mentioned the gear ratios earlier. That's again, the easiest & cheapest way to increase torque to the drive wheels.
My only argument against it - Are you confident that your truck will STOP the extra weight? Will your truck HANDLE the extra weight, in terms of emergency manuevers...
Those two things are certainly worth thinking about for people towing.
Sure, the 1/2 truck, especially with newer 6,8,10sp automatic transmissions, may get everything rolling OK.
But, will the 3/4 ton or 1-ton STOP the load better? Especially if there's a trailer wiring/plug/brake failure... Will the 3/4 or 1-ton HANDLE the trailer better, under those emergency braking or emergency lane-change scenarios?
People ignore emergency situations unfortunately, but they may be the biggest reason to "upgrade" to a 3/4 ton or 1-ton truck.
Anyway, enough rambling there...