As has been mentioned start in the UDS thread. You can move pretty fast thru a lot of the posts.
This is one of of those every man/woman's UDS will be slightly different.
All depending on needs and wants. The basics are the basics, there are a couple of manufactured UDS and you should be able to get an Idea from their web sites. I really don't know of any plans as such other then back in the day. In the POP MECH magazines and the handyman ones.
I don't think, you can find a better forum than this one for answers to any of your questions.
Since every ones skill sets are different, my instructions might not be usable to you! Welders, plasma, torches lifting devices and power tools from here to over there. So best thing is since you are retired.
Do what I do spend some time on this site and cover the UDS thread and it won't be a while before you will pick up the Lingo, and the subtle differences between an this one and that one, and the one over there with 6 wheels, running water, a towel rack and a hot tub up near the tongue
It is confusing, and some what intimidating, and I have built a couple of different drum types and a large portable on a trailer for my son.
Being retired I know the feeling, but it will pass as the honey do's quickly fill up your time
Relax and enjoy the forum, smoke is slow!