Personally, I would go with a whipped dog sight unseen deal, a shave ready from the hobbyist section. or a shave ready offering from B/S/T. I would not focus on brand name or the like. Get something that you know will shave your face. If a straight is for you, you will soon find yourself wanting to invest in hones and then your choices are limitless.
My first straight is still my only straight. A Weck Sextoblade, and if I had to do it over again, I'd make the same choice. Based on my experience with pocket knives, I don't see myself taking up honing. That leaves disposable blade straights, and the Weck gets the vote as the most traditional of the bunch.
Probably a vintage quarter hollow or near wedge from Solingen or Spain depending on my budget. I think the heavier grinds are more forgiving at the beginning.
Having had poor experiences buying on line I would try to restrict myself to what I can see in the flesh before purchase. In my market area that means Dovo. I have added eleven to my rotation this year and they have all been great shave ready razors. If I could only get one it would be a Prima Klang blade and I would select the one with the most attractive wood, to my eye. Prima Klangs are ground and honed by their masters. This would assume available funds. The expectation would be it is a life long purchase, which after fifteen years my first Dovo has been. In my experience, Dovo makes an attractive, well assembled, and hard wearing product. Takes and keeps an edge with minimum effort. If I did not want to deal with maintenance issues I would get a Feather Artist Club DX. I have used, very happily, Dovo shavettes on and off for years and like the product. Just tried a Feather AC DX and the design and ease of use is simply unreal. Paired with Professional Light Blades it is a cinch to use. With experience you can move up in blade exposure and thickness to transform the razor. Again, I would view it as a lifelong purchase and the Feather is up to the task.
I would follow the general advice and get a Dovo Best Quality 5/8 full hollow ground with a round tip and the poorman's strop . I have found most of the new straight razors are not truly as sharp as I like them to be. The poorman's strop comes with a simple balsa and 2 fine chromium oxide pastes to make the dive into str8rzr shaving a much more pleasant one; especially if you are coming over from using a DE blade.
Tough question for a lot of us! If I could only have one it would have to be my custom Zowada. Way to pricey for a first razor, but knowing what I know now, I would choose it over all others.