Well, first off, you can buy regions for less than that from someone who already owns some regions; there's a lot of folks downsizing their holdings these days.
You can buy as much land as you want to pay for. And pay tier for... The # of parcels depends on how YOU divide it up.
As for "will land rent fast", that depends on a -lot- of factors. One is your asking price, another is amenities. I know a guy who owns 3 regions, he's a decent guy. His rent price is dirt cheap.. and he has trouble keeping his places full. One of his big issues is that he has no clue how to do nice or even halfway nice landscaping. He completely flattens out the land all the way to the edges, puts a sand texture on it, tosses the houses on it every which way, and thinks he's done. It's not very nice looking, not once you've seen what can be done with SL landscaping. (go wander round Botanical or Alchemy Immortalis sims sometime, among many other places I can think of that are just lovely to look at..) He also does not allow people to have their own music in their rentals; each house would have to have a separate plot and he has mostly apartment type buildings, which don't work so well with that.
Another lady I know owns well over a dozen regions with a central theme, lush landscaping all around, has varied free events every week or so at a community center she built in the heart of her sims & open to everyone, and there's a great mix of residential and commercial land sites. She is an absolute sweetheart too, always willing to work with someone on anything you need, whether it's terraforming help, adjusting the borders of your plot so the house you just bought will fit, even letting you pay your tier a day or two late without reclaiming your land 10 minutes after the tier meter expires. Absolute angel. Most of her residential properties are double prim too, which is hard to find sometimes. AND she's very reasonably priced, more so than many landlords I've seen. Yet she has trouble finding renters who will stick around. I have no idea why, unless it has to do with the glut of land on the markets and people leaving SL in general.
Speaking only for myself, I love SL.. But sometimes I think about leaving too. Thanks to the lousy economy, I've been out of work for a couple years now. But I like owning a little bit of virtual land and I love SL despite its flaws and I adore the friends I've made in world. Sometimes it's really hard to choose between paying my tier and paying RL bills.. And the bills usually win. :-\
of course, your mileage may vary.