Hmmmm, I don't have all that much experience with the T4x series.
My first Thinkpad was a T40 that I used for a few years before passing it off to a family member - not sure where it is now. From there, I used an X31 for several years before passing it along as well - it is still chugging along. From there, used an X60s that is still my traveling machine which has survived drops, hot tempertures, planes, trains and automobiles. My X61 and my wife's T61 do not travel much.
The processing requirements are quite low - microsoft office, web browsing and playing DVDs for health demonstrations. T42 or better should be more than adequate with a Pentium M.
Battery life is key, no doubt.
The rough and tough part comes in transit and from inexperienced users doing foolish things. In typical use, the machines should be sitting on a desk or other reasonable surface.
The current equipment in a similar role is a Tecra M2 and a Lenovo C100 as well as an X22. The M2 was recently taken out of service due to overheating after three years of use (after being revived from my company's recycle pile).
For me the travel is not too different from a student riding a mountain bike around campus.
To extend battery life, I have pondered the following - http://www.tekkeon.com/products-mypowerall.html
or perhaps a solar panel. Around $120 a pop - probably need one, but would like three.
I'd want six good primary batteries to maximize useable computing time.
The requested budget over the next few years was anticipated by others to be $1500. There is around $1000 that might be available in a gift designated for things such as this, but I may not be able to get it all.
Needed software is available at little to no cost for our purposes.
So let's say I have $800 to spend to start. There is already one T43 in use and one T43 and one T42 available. I also have a T60 with a line of dead pixels across the screen and a dead battery in need of replacement.
One external battery plus 5/6 batteries would probably be $400 of that budget, leaving $400 for machines - assuming $90 each for good T43's complete less batteries, I get four machines. Assuming $125 for T60's, I get three machines.
Could I beat these numbers from someone with really good batteries in basic T60 machines? I'd love to.
Can I get five (or six) well equipped, complete T60's with IBM graphics and nearly new batteries and ac/dc chargers for $600 (leaving room for one spare battery and the external battery)?
I've bought effectively bare bones T60's for around $100 and have piles of 512MB pieces of DDR2, but five complete machines for $120 each seems like a stretch.
Then again, five $90 or so T43's will cost just a little less but be a big step down in machine.
Should I float a want to buy ad in the Sales Forums with my budget and specs?