Your going to get a whole lot of differant thoughts on this one.
As humans we tend to work in our comfort zone, so if you happen to have bought one brand of tool, we think that ones the best.
One free way to do some reseach that I've used is to go on Amazon.com look up the the tool then go to the bottom of the listing and look at the reviews from people that own or have bought the tool.
A few tips:
It's almost always cheaper per tool to buy the set of tools not just one tool.
When you buy the set it should come with a charger and 2, batterys.
General rule of thumb:
A 3/8 drill will spin faster then a 1/2 drill.
A 1/2 drill will have more torqe so they can have more power to drill bigger holes.
I would not go with less then 18 volts. What good is a drill that does not have the power to do the work.
Any of the major brands, Milwaulke, Porta Cable, Ridgid ECT. all make fine tools.
Black and Decker and Skill make more of a home owner light duty tool.
Pick one brand and stick with it so all your tools will run off of one battery style.
Ridgid offers a life time warrenty, even on the battery, which can be a huge savings over the life of the tool since some of the batterys are so expencive.
I own several differant brands of drills and range from a 3/8 Milwaulke to a 1/2" Hole Hawg. My go to tool 90 % of the time is my impact driver, not a drill. I can drill, remove screws, install and remove nuts and bolts, install sheetrock all with one snap on attachment.
May want to check out CPO Tools. They have a lot of factory reconditioned tools at a big discount.