Can I use any audio recorder for filming?
I guess it really depends on your production and how high of quality you want. You will hear countless times, if you stick with this hobby, that audio far outweighs video when it comes to importance. This is somewhat counter intuitive because the video is much more obvious an impact than the audio. For this reason, however, purchasing the best audio solutions one can afford is usually the best solution (depending on your scenario, of course). 

Recording inside a car is a whole new can of worms. They're incredibly noisy, especially while operating. For this reason, sound engineers will often entirely reconstruct car scenes specifically by recording sfx, foley and performing ADR with the actors. Hopefully you can think of some creative ways to use your car while it's not operating because it's incredibly difficult to capture usable audio. 

Essentially you're asking the question that every beginner videographer asks: "what's the cheapest, all-around best audio solution I can get without having to think too much." It's a fair question, but one that doesn't have the best answer. I don't think a small palm recorder/ipad is a good idea. What I might recommend is a combo solution. I would pick up a wired lavalier mic (atr-3350) for $20 dollars and plug that directly into your camera for portrait shots, and then I might pick up a camera mounted shot gun mic (Azden SMX-10) for $80 for wider shots and you'd have a good universal, cheap solution. 

It's worth noting that plugging microphones directly into Canon cameras is often not the best audio solution because of the low quality preamps and AGC found on the camera which results in adding a "hiss" to your audio. You can clean this up in post but it does degrade the audio slightly. Something to look out for, and something that you can kind of fix with Magic Lantern software you asked about earlier.  

When you say "exterior sounds" I assume you mean dialogue (correct me if I'm wrong). For outside characters tend to be on the move, so having them attached to your camera (a la wired lavalier) is a bad idea. You never want to leash your actors to your camera for obvious reasons. Additionally, clothing will rub against the mic and create unusable audio. If you're not in motion, the only thing you need to worry about is wind. A few little dead cats for your lavs should solve that problem (while making them more noticeable). However, the ultimate solution to outdoor dialogue is a shotgun microphone on a boom pole. But, that is probably out of your budget, so a cam mounted VideoPro mic will do "well" as long as you get it close to your subject.