First, since you have blobs for iOS 4.3 and you have an iPad1, you can restore that iOS 4.3. back onto your iPad. To do so, follow the steps in this sticky:
http://www.ipadforums.net/jailbreak...815-how-downgrade-your-ipad-ios-firmware.html
After you've restored that version back onto your iPad1, then you can browse to here
JailbreakMe 3.0 (using
only the Safari browser on your iPad1) and follow the two-press steps to be jailbroken. Easy peasy!
Sorry for the earlier confusion - I missed the part where you asked how to downgrade/re-jailbreak...
Now, for your last question:
You can stay on that iOS (4.3) as long as you wish. That's kind of the "golden ticket" firmware, in that it's the last version (well, 4.3.x is) that the jail breaking community was able to do an
UNtethered jailbreak. Means you don't have to worry about being near your computer if your iPad needs to power up or reboot.
However, for the iPad1, there is a tethered jailbreak, on both iOS 5.0 and 5.0.1. A lot of people (myself included) have moved up to that version for a number of reasons, mainly because we can
D) and also because that's the direction apps and tweaks are headed. Most developers are making their apps (and Cydia tweaks) to be compatible with iOS 5, which does cause those on iOS 4.3.x some issues.
Only you can decide if you want to run a tethered jailbreak on iOS 5.0.1 or stay at iOS 4.3 untethered. The good thing is there is no pain for you if you decide to try the iOS 5.0.1 jail break. If you find you don't like it, or it's too much of a hassle to be tethered (disclaimer: I've had zero trouble with my tethered JB), then you can always go back to iOS 4.3.
Long story just a tad longer: You don't ever have to move to iOS 5. Or, you can wait, while on iOS 4.3., to see if an untethered jailbreak becomes available and not deal with any potential hassles with the tether. It's entirely your decision. I just hope I have helped - and not just muddied the waters.
Good luck with your downgrade and do let us know how you get on (or have troubles).
Marilyn