start with local dealers and try out as many machines you can---put together practice sandwiches for quilting- and as you try them out the questions will come to you-
ask about (free owners classes) many dealers offer them- which are great to take advantage of because they teach you all the great features of your new machine-so you are able to use it to it's full potential.
ask about service plans- (my viking dealer gave me free cleaning/service check ups for the first 2 years)
ask about warrantees, - if you don't know much about the manufacturer=company ask about their history---
get on line and look them up- read comments and reviews-
check with other quilters at your lqs- what are they using?and why?
i think it is always best to buy locally- then if you do have problems-technical issues you can get help-