Well, the system you described certainly provides what you will need, but as you suspected, there are some things to be aware of.

If you want my honest opinion about the Roadmaster: stay away from it. It may have a lot of useful features, but the quality is very subpar.
The biggest shortcomings are the signal to noise ratio, the warranty, and the preamp voltage level. The SNR is only 80dB, which is very low for a car unit and involves a very audible level of noise in the system. You should get a unit with at least a 90dB SNR, and higher is better.
Also, from reading the user manual, the warranty is only 30 days. This instantly raises the red flag as to the quality of the unit. A good unit is generally warrantied for a year or more.
Finally, the pre-amp voltage level is only 1V. This translates to a poorer signal to your subwoofer amp. A good head unit should provide pre-amp outputs at 2V, minimum.
There are many units for less than 100$ that give you SNRs > 90 dB and many other things as well.
I would strongly recommend getting a unit such as this one:
http://www.cardomain.com/item/PANCQC1333U
for example, because it is a name brand, comes with a full year warranty, has a 96dB SNR, 2 x 2.5V preamp outs, MP3 playback, and a front AUX input. (also free shipping from that site, and a free wiring harness and install materials)
The choice is ultimately up to you though. Just make sure to be absolutely sure of things before spending money. It may seem like a larger sum now, but it will be a good choice down the road as you won't have to spend more money replacing cheaper parts that may have failed.

As for the amp on ebay, the price alone makes me very wary...however, for $30-something + $24 s/h, it's not a huge loss if things don't turn out ok. By the way, the 300W rating is peak power, which is an easily-falsified and very inaccurate rating of power. Many manufacturers bloat this number even though the amp may not actually be all that powerful. RMS (@ 1kHz) is a moderately better rating, and full band RMS is by far the most accurate power rating.

EDIT:
May I recommend getting a Pioneer amp instead of the Legacy amp:

300W 2 channel Pioneer

The pioneer amplifier ratings are CEA2006 compliant.