Some time back, I put together (with help from a couple of other drivers) a presentation on how to use data acq systems for driver training. I probably oughtta put it on my website...
Anyway, I use an AIM Mychron3 XG Log. I chose it because of cost (under $1k), good software for data analysis, and ease of installing other sensors of my choosing.
Best no-limits system? I've never worked with Pi or the like; however, I do work (at the day job) with systems costing over 20-30k. However they're for somewhat different purposes. I would expect that something like the higher-end RaceTech systems would be it... I guess it's more features you're after than anything else...
For top-end stuff, you want systems that can log at 50Hz (to track suspension movement for shock tuning), CAN bus interface to plug in and log signals from your engine ECU (not that we have those), as many input channels as possible, as much memory as possible, and preferably the ability to plug in custom sensors (so you're not locked into buying only the sensors from the data system mfgr.). Burst or even continuous telemetry capability would be nice too. Data download style can relate too; if you have the option to just swap out memory cards (rather than plugging in a cable to a powered-up system), that's faster. The more segments the track can be split into, the better. Predictive lap times are nice too.
For even the basic system/budget club racer, I'd want to at least log speed, RPM, lap times of course, and lateral G's. The quality of the data analysis SW is also relevant; you want something you can understand well enough to use and get good results out of quickly. I would definitely recommend a GPS-based system, like Racepak; mine isn't, I got it before they became widespread and good, and I still futz with the beacon timing as a result. Next one will be GPS-based.
The display is also relevant. I prefer the style that can be mounted on or behind the wheel (mine's right behind). The two most important features to me are to know rpm's and lap time; specifically, having a programmable shift light feature allows me to ignore the tach. Instead, I just drive till all the lights flash, which is easy to see while watching the track, and shift then. Having lap times tracked allows me to do a couple of things. If it's a qual session, I can track how I'm doing, and can park the car when I know I've got the qual time I need - particularly useful if I'm just about to hit traffic or trying to conserve equipment. During a race, knowing my current (rather than best overall) lap time allows me to track how I'm doing, if I'm holding my pace, if I can chase down the guy in front or if I know I'm going fast enough to hold off the guy behind. Knowing I don't have to push faster than I already am means I don't overdo it in a corner I'm already maxed out in, therby driving off and stuffing the car or losing spots.
Seems like you should be able to get a good, GPS-based system for under $1000.
Additional things that are nice to have - knowing braking and throttle points are good, so if you can add those on to your system one way or another, it will be useful once you start to become familiar with the basics. They all usually come with temp and/or pressure sensors, but those are more useful for monitoring your engine (oil press, temp etc) which is probably not so much what you're after with the system now. Knowing when you're on the brake, when you go full-throttle, and how consistent those points are will become more useful.
As for steering, I've had it for some time in my car. It's barely useful for me,, and will just not be something anyone less than an expert is going to get enough out of to justify the expense (of the sensor). So instead, I would recommend (if you really want to see what you're doing with the steering) - use a videocamera. Ideal would be a dual PIP system like the Chasecam setup (or the one I have) that allows you to both see the driver steering input and the track in front. This would allow you to correlate driver inputs with the track (and possibly what's going on out there).
That's a start, hope it helps. I'll post back if I get my presentation and notes uploaded. It used to be on the Waterford site, but they've dropped the link.
Hopefully this year I'll have more time to spend learning how to use the darn thing properly. I would be happy to share data with you though Ray. 
