Welcome! Good choice to get on the dirt (a lot less cars and trucks here.

) Of course, the trees make up for it.
Do you see yourself as an aggressive rider or do you see yourself as having the potential to be an aggressive rider? Not so much a "Joe Racer" type, but the kind of person willing to go a little faster, higher, or farther every time you go out? Or, are you looking to just casual ride, but want to make sure you have enough power/suspension for those here and there hills and other obstacles?
Assuming you've settled on Honda and on one of the CRF250/450 models...here's my opinion:
- Either bike will fit your body dimensions fine.
- If you see yourself as someone who will, most likely, be a moderate/take it kind of easy on the trails, doing trail level jumps, NOT riding on long open fire roads (long high speeds,) NOT doing HUGE hill climb type stuff and, generally, just out for the "fun" of it...a CRF250 will be perfect. Those bikes have plenty of power and suspension for the occasional hardcore beating for sure.
- If you see yourself as aggressive and doing things that pretty much surpass what is listed above, you'd probably want the CRF450.
X or R...that is the question...
- The X's were made for the trails. They have the e-start, softer suspension, headlight, kickstand, etc. They do require their share of mods to bring the bike to life (power-wise,) but the mods are generally cheap and easy to do yourself. The busiest forum here, by far, is the 450X forum. I have not owned one, but they seem to be pretty nice bikes. The X has a wide-ratio tranny that can be better in the trails.
- The R's were built for MX racing. They have only kickstart, stiffer suspension, no headlight, no kickstand. They are a lighter bike and, typically, do not require any power-related mods for riders like you (and me.) The R was not designed for the trails, but can be a pretty good trail bike with very few changes. The R has a close-ratio tranny that can sometimes bite a bit in the trails. This can be overcome with some gearing changes. The R (right out of the box) tends to be a "staller" in the trails. Again, this can be fixed with various approaches (idle speed, flywheel weight, etc.) that are free or cost next to nothing.
Both bikes have they're own little gotchas and you wouldn't be wrong going with either flavor of 250/450. A popular product out right now is the Rekluse. It seems to be the shiznit for all flavors of bike. I don't have one and don't have intentions of getting one any time soon (not for lack of want...it's just that there are other priorities right now.)
I am also a new 40-something with most of my M/C experience on the road (casual rider.) In '04, I bought myself a CRF230 and my son a CRF150. This past May, I bought an '06 450R and recently put my son on a KTM 200 (2-stroke.) Would have probably put him on the 250X but he wanted to try a 2-stroke and I had a buy I couldn't pass up. My son had ZERO experience on a motorcycle (of any kind) when we bought the CRF150 in '04. In all honesty, I might have even considered a 450X for him (so we could have an X and an R in the stable,) but, in my opinion, he doesn't have the body weight and general motorcycle experience to run with that bike (6'1" / 140lbs.) This might clue you in a little on how you might advance faster than you think.
If you intend on riding regularly, you have a competitive spirit (always looking to go faster, higher, farther than the next guy)...within your comfort level of course...and you will be riding in an area that might have some decent hills/rocks/power sucking obstacles, you'd probably be best to go with a 450. As far as whether it is the X or R...that will have to be up to you. I will admit there have been times I wished I had the e-button, but overall, I doubt I would ever go to an X. Plus...the R's are cheaper.
Again, this is only my opinion and it's worth exactly how much you paid for it!

Best of luck on your decision.