What is the ideal setup concerning performance and HDD longevity? What should I have on my laptop's hard drive and what should go on the external? Everything on external? Running Reaper and the plugins from laptop and recording to external seems like it would be ideal? I could set it up to run everything completely from the external and keep a copy of the interface drivers on it, then I could record on anyone's computer after a quick install

The main thing is to reduce wear as much as possible on your laptop drive by sending the really big writes and erases somewhere else(the external). The biggest thing to put on the external, is just the project files, that all of the individual recorded tracks record to.

The laptop will still be using its drive for the operating system, and for RAM overflow(the writes to and from these two things are the main reason why the additional writes for a recording project put too much stress on the drive - it can only do so much at one time. Make it do more and it will get hot. When hard drives get hot, often, and stay hot for extended periods of time, that is when they will fail. This is all that recording audio projects to a system drive does, all the time.), and shit like that, but whatever you can send somewhere else is good. This mainly just means the project audio files.

I would probably keep the DAW installed on the laptop. Really, in the case of that, i don't think it matters a whole pile, either way. Especially if you are using Reaper, which is designed to run off a flash drive if you want to. You could put it in the laptop, or on the external and probably not see any performance problems(unless the external drive or the laptop USB is 1.0 or a buggy implementation/chipset). I just like to have my program files all in one basic place(in this case, on the laptop) sort of as a habit.

What Chris said. Send the actual recording itself to the external, preferably something fast. 7200 RPM USB 2 is perfectly acceptable though, for anything you're likely to be recording without a setup that would demand something else fer yer rig anyhow. I prefer Firewire, but that's just me and am totally willing to rock USB 2- or 3 for work porpoises. I've had no issues with USB 2. I have had issues with the internal drive on a Macbook Pro.

This is especially critical with a laptop, but recording to a shitty 5400 RPM drive on a desktop is still No Fun At All and can Fuck Right Off. Weird digital errors abound and will Bugger Your Audio if you're bringing in a bunch of tracks at once. Probably not such an issue with newer computers (the lack of 5400 RPM drives in desktops and all), but still. I'll track to the system drive sometimes on my Snaxmount Rackintosh but that's it's own animal and very much Not A Laptop unless you are actually physically Cam-Sized in the figurative sense, as the literal d00d ain't that big.

In short, if you have a drive with a decent speed and connection and it's not the one with your OS on it, record to that and you should be solid. If you're rocking two dozen tracks in at once with the rack of stuff to get you there you shouldn't be using a laptop anyhow.