Helmet: DOT is a 'minimum' for most areas with helmet laws. ECE is the same for Europe. Snell is a third party testing system that is sort of a "+1" to the other two ratings. You can read a bit more about the differences here:
http://www.webbikeworld.com/eicma-20...-standards.htm
Here's my advice: Get a helmet that FITS WELL, is either full face or a high branded modular (e.g. Shoei, Schuberth). Find a good gear shop, put the helmet on, and leave it on for thirty minutes. Move around, how do you see? Is the helmet tight enough it won't move if you shake your head around but not so tight it has pressure points? That's your sweet spot. If you have one piece of gear you can overspend on it would be the helmet.
Jacket: thickness really isn't the primary measurement here; fit and quality are more important. It needs to fit tight, like the helmet -- this shouldn't be something that fits like a street coat. The armor needs to get over the protection spot (shoulder, elbow, etc.) and stay there as you move around. It should feel a touch awkward to move in. Leather is good for abrasion, but make sure it has -- or has pockets to add -- impact protection. For a street riding piece I'd get something with a back protector integrated. The back protector is usually focused on protecting the spine and other vitals around it. As for quality look at the sewing. You don't want any loose threads or popped seams: those could tear the jacket apart in a bad situation.
Another good site to look up gearwise is revzilla. They have a good reward program, excellent customer service, and their youtube channel is a great research tool.
My favorite brands are Rev'It and Dainese for jackets and such -- they're super high quality brands that are excellent for me due to my tiny size (they're European, so they run thinner/shorter). My favorite helmet is Schuberth because it fits my head 'just right'.
Strong "quality value" brands might be A* for jackets and Shark for helmets.