At 5'3" I think you will like the size of the blast. They come with a low seat option, which is probably what you'll want to get.
If you are buying it brand new (hence no chance of it having been a Rider's Edge training bike in its prior life) it will be fairly reliable. The Blast has been essentially unchanged for 7 years, with the only 'updates' being fixes working out the kinks of the model. So its a pretty solid bike.
A few items you might take issue with:
1) Oil. If you run it hard, oil will mysteriously disappear. It blows out the airbox, but in such small quantities that you don't notice. As a rule of thumb, I check my oil every other week, and sometimes I have to add a little. I ride easy, so I don't really have much of this problem.
2) Vibration. The bike will vibrate more than your Sportster. This is noticable at idle, mostly, and at hight 75+ speeds. You do get used to it. I barely notice the vibration anymore.
3) Tires. Expect your rear tire to need replacement more often than is typical for a motorcycle. Depending on your riding, a rear tire will last 4-5k miles. The front will last much longer. Replacements for the rear tire are $200 at the dealer (with labor/parts/tax), so figure that into your regular maintenance.
4) Top Speed. The Blast will top out at 85-90mph depending on conditions. Its not a fast bike, but it will get 70mpg so I think that more than makes up for it (the fastest highway limit is 75 anyway).
It is a light and nimble bike, very easy to handle.
There is a yahoo group dedicated to the Buell Blast that has alot of information, and knowledgable folks that can help you solve most problems you may encounter. Here is the link: http://autos.groups.yahoo.com/group/The-Buell-Blast/
And if you take off the flyscreen there isn't much difference in wind/ride style. I think it looks much better without the screen.
I wonder if anyone else on this forum owns a Blast?