1) You would need a good relationship with that dealer/programmer. 2) You would need to get all the things you are going to want in your system installed and ready to go.
Otherwise it is going to cost a ton of money, if you keep calling them out to make changes in the programming as you make changes in your system.
I would question why you wouldn't be buying the equipment from the dealer/programmer however. Any savings you would get purchasing from some grey site on the internet, could quickly evaporate if you run into any problems. Not to mention the very 1st point; building a 'good relationship' with the dealer/programmer.
The hardest part about this, is finding a good dealer to work with. There are soooo many shady MF'ers out there doing 'control systems'. That really have no business in our industry what-so-ever. I constantly have to fix BS I see in the field by these other 'so called' experts. It leaves a serious black eye on this industry, because almost universally, the customer blames the remote as being some junk the 'so called' expert sold them and 1/2 arse programmed. When the whole problem is the idiot that installed/programmed it, just threw it in and ran off with a check.