Before buying your own system, I will sugest that you get a hosted solution for few months.
There are several companies that will rent you billing “online” for as low as $100 dollars a month.
In six months, you expent $600 dollars, got a lot of experience about what is good/bad about the billing, and still not “trapped” into a system.
And you get the support for those critical months that you are on the learning curve, when the gateway goes online and you run into those pesky troubles.
Or the other option is expending thousand of dollars, and hoping that the sales people do not sell you something that you do not need.
Just my $0.02 !!!!!