I'll take the opposite side of the popular argument. if you have the funds, just go buy the bench. It won't be your ideal bench, but you will have it and be able to work on it right away. If you build your first bench (and especially if you solicit opinions from people like us
) you will spend 3 months agonizing over the details, 3 months drawing plan after plan, and then 6 months to a year (or more) building it. And it still won't be your ideal bench.
If you buy your non-ideal bench, you will get to start working on it right away and start learning right away what you don't like about it, so when you do build your ideal bench, you'll have a better idea of what you really want, rather than guesses and a thousand opinions from us internet folks. But, and this is the key but, you will actually get to do work on it, rather than spending the better part of a year or two thinking about your bench.
As an alternative if you really want to build a bench (and hey, who in this forum doesn't) take a simple, easy, fast, and cheap plan and build it in a couple weekends, again for the same reasons above. Don't bother with whether you want a Scandinavian bench, Roubo, Nicholson (whatever that is), or any of the other names that get thrown around, you don't know yet what you really want from the bench, so go for something fast, easy, and proven. It probably doesn't matter which one, as long as you can beat on it and clamp stuff; that will get you started, and do most of what you need it to do.
Myself, I of course didn't take any of this advice
My first self-built bench was the FWW #4 Tage Frid bench, not something for a beginner. But, when I built it, I had used several benches in various shops, and knew that was the bench that I wanted (and it was; my main current bench is a lower version of that bench). I also had use of my non-ideal but adequate benches (or bench-like adaptations of things not meant to be woodworking benches) so I could still get stuff done. Despite following a fairly detailed and comprehensive plan, it still took me several months to build it, and I was fairly experienced at that point in my woodworking life.