That's a decent HU. It is one of the models that you can change the greeting on. $50 isn't too bad, but it is actually $66 shipped, which, IMHO is a bit on the high side.
I got my T1807 (same model as this one) for only $34 shipped in nearly brand new condition. I think you could get one cheaper if you kept looking for awhile. Don't be rushed to purchase; there are a ton of these for sale. If you're patient, you should be able to get one for about what I did.
For tons of info on the T1807, search this forum with that #. My T1807 install thread is stickied in this section.
As far as adding a sub - you would have to use the rear speakers for the sub input, as this (and most all Scion HU's) DO NOT have line-level outputs. The most recent Scion HU's have started adding line-level outputs, but they're expensive HU's.