Go for it. Top end budget would be $100, unless it's something really special.
The main ones I had in mind were two of the three mid-90s big guns: (you may need to look on ebay to get a sub-$100 price)
8880: The best of the seven large-scale cars they've made by some distance, with steering, drive and suspension on all four wheels and a transmission system.
8480: This one probably has the most complex gearing of any model ever. It has two motors and the old FOS lighting system. Unlike most sets, the alternate model is also very good. It used to be very common on ebay and Bricklink two or three years ago; it's less so now but can still be found for about $100 on ebay.
I would have also recommended the third one, the 8479 Barcode Truck, but the Brickset reviews indicate that there may have been some bad QC with the controller unit. I ran into some problems with mine last year, although I was able to fix it. It's amazing to watch in action though.
There are also the two Control Center sets, 8094 and 8485. They're cool, but expensive and worth getting more for the parts than the models. You are better off buying the above stuff first. There is also the 8466 4x4 Off Roader, a newer set that has the 8880's functionality in a monster truck frame. I actually have an extra unopened one of these (got it for free in 2001 due a freak error on Lego S@H's part) that I will be selling soon.
If you're interested in other things that aren't road vehicles, the 8839 Supply Ship and 8425 Black Hawk are also very nice sets for about $40-50 each.
wooo! 8862 secured (postage/ebay seller not being a t**t pending)! £35 in total (circa $70 IIRC)
damn, CP, I could really have done without another drain on my limited funds 
Excellent.

You might be surprised at how large this is. It's much bigger than it looks in the box/manual pictures.
I don't want to think about how much I've spent on this stuff over the years.

8438 - I'm thinking this is ripe for modifying
This is actually a second reissue of an older set, the 8460 Pneumatic Crane Truck. I have the original one. It's a great set, but note that the newer crane, 8421, is quite a bit better. The 8421 is a current set and priced a bit over your range at $150, but it's available at many retail stores and you may be able to find it on sale somewhere.
8891
This contains some cool ideas, but you can read the whole thing on that site I linked to earlier, so it's probably not worth buying separately. All of the models use pieces from sets around 1990, mainly 8862 and 8094.
I always figured 8868 was a little odd. Not the model but the concept. Why would there be a grab crane on a truck chassis ? The crane looks too small to really reach anything.
I'm not exactly sure what it's supposed to be, but I have seen things like it in real life. I think they're used for lifting tree logs.