There are a variety of questions and answers that can go on here, but there are a few simple things that you need to consider first. First and foremost is WHY you want to put all this equipment on your car. It seems to me, that someone that is just getting into chasing should be more concerned with learning how to chase rather than being concerned about collecting data. Granted yes, there are a variety of benefits of having particular pieces of equipment on your car, but it comes at an added cost of difficultly. Maintaining your equipment, making sure that it isn't going to go flying off your car at 70 mph, setting everything up properly, it's a lot to deal with when you're still trying to learn. You don't need a bunch of equipment on your car to make you a chaser, unless you're only doing it for the attention in which case you're doing it for the wrong reasons.
Secondly, if you are dead set on have the equipment, then you need to decide what you want it for. If you just want it to kinda play around with and don't want to spend too much time or money on it, then there are a variety of options available. If you're a little more serious about things, and actually care about about the data you're collecting, then there are several things that you need to consider that are going to complicate things more.
And for the record, it's hard to say with complete confidence that ANY instrument is accurate. It's a loaded statement. Even so called "professional" sites like the OK Mesonet have their share of issues that need to be considered. Of course this is all my own opinion, you can take it however you want.