My basic rule is that you never regret quality. Buy a little more bike than you can comfortably afford and - if you keep at it - you won't regret it.
That being said, you kind of need to decide what kind of riding you want to do, but it's OK to change your mind.
It's also OK to have more than one bike for more than one purpose. For instance - your beginning comfort bike for commutes, a weightless wonder for centuries, and a big ol' recumbent for cross-country tours could share garage space at your place. (What? Cars in the garage? Never heard of such a thing!)
If you are worried about whether or not a bike can stand up to your weight, just swallow your pride and ask. If your LBS is any good, they'll level with you and steer you in the right direction. (Been there, done that. It didn't kill me, or even hurt over much.) You'll weigh less, and be more fit the next time you wander in. The bike guys will probably even be rooting for you. For sure we will.
Give big space to the festive dog that make sport in the roadway. Avoid entanglement with your wheel spoke.
(Sign in Japan)
1978 Raleigh Gran Prix
2003 EZ Sport AX