There are already many sites, books and articles that will tell you how to choose a rig, and how much fun it is to operate QRP. Choosing a rig is easy; having fun with QRP requires the right state of mind and being in touch with reality.
QRP Is Different
Operating QRP is not "just the same" as operating with 100 W or more, only with less power. Yes, very often, someone using a 100-W transceiver, or even higher power, could work most of the same stations at QRP power levels. But not always. Assuming you can reduce your power output without affecting the number of stations you work, and how long your QSOs will last, is the first step toward serious frustration.
While your QRP signal is only about two S units weaker than a 100-W signal, at this point at the bottom of the sunspot cycle, those two S units can make a bigger difference than some QRP fans would have you believe. It's good to encourage hams to try QRP, but giving a false impression of what might be done may drive off someone who would have stuck around were the realities revealed at the outset.
QRPers often aggravate the signal-strength situation by using less-than-great antennas. Portable operation is a lot of fun, and it's exciting to make contacts with a tiny radio connected to a short hunk of wire tossed over a branch. You can make many contacts with such a station; I do it all the time. But you will hear many more stations than will hear you.
Know Your Limitations
In The Joy of QRP, Adrian Weiss, WØRSP, tells of a new QRPer who wrote to him in frustration, because he could not work most of the stations he called. Ade pointed out that with his antenna, he should expect to work stations within a few-hundred-mile radius of his location, even though he could hear stations farther away.
For example, when I operate portable, I use one of two general types of antenna:
The second antenna is much easier to put up and take down, but I usually find I have lots of time for that because I don't make many contacts. At various times I have used a fire escape and rain gutters for antennas too, and even worked some DX, though not with QRP. We hear about hams who worked amazing DX with incredibly poor antennas only because such occurences are exceptions. If they were commonplace, we'd never hear about them.
While we'd all like to work far-away stations, there is much pleasure to be had in working stations closer to home, especially with QRP. So don't get frustrated. Work the stations you can work, and throw a call at the far-away stations once in a while. And always be on the lookout for better antenna possibilities.
More About Antennas
After the operator, the antenna is the most-important part of a station. A good operator makes contacts under poor conditions, even with marginal antennas. Good antennas don't have to be complicated or expensive. As QRPers we limit our power output to 5 W; the only way we can increase our signal strength is with better antennas.
For a year or so in the 1970s, I had a three-element triband yagi, about 30 feet (9 m) high. I worked a lot of DX with an HW-8 back then. Before and since then I have always used wire antennas at HF. I like wire antennas. They feel organic, and they are inexpensive. Like any antenna, though, wire antennas work better when they are in the clear, away from buildings and other obstacles, and up high.
"High and in the clear" was easy in the days of large suburban lots with several tall trees, and before the days of restrictive covenants, ordinances and other infringements on our right to work DX. So many hams live in places where outside antennas are strictly forbidden; I'm one of them. As QRPers, we have options: We easily can get out of the house!