The spring rate of the bars doesn't change, although they can sag over time. This however can usually be adjusted out using the torsion bar adjusters at the lower control arms.
There are other things to look for though, you want to keep an eye out for any damage mostly. Deep gouges, scrapes or other damage can lead to failure, and torsion bar failures are pretty spectacular. You'll also want to check the torsion bar anchors in the cross member, taking a look for any rust in the cross member and for good welds on the anchors. There's a member here who recently had his torson bar anchor tear loose from the cross member. Its a lot easier to reinforce the anchors BEFORE they tear loose, as they have to be clocked correctly.
And finally, if you're re-doing the front end, I'll assume you probably want the car to handle better? The stock torsion bars have a pretty low spring rate, resulting in a very undersprung front end. Most folks are a lot happier with torsion bars in the .92-1.00" range, I usually recommend 1" bars for a more modern ride and much improved handling. Just my .02.
