What do you look for in a followspot for footcandles at your target area? Granted, this is based on numerous things such as ambient light, but is there a certain footcandles number you desire or require from a followspot before you'll even consider using it for your application?
Following up on @RickR 's post. How bright is your wash at full? When I run my wash at full I'm up around 95fc. So a 100FC spot isn't going to do much. Find a photographer friend and borrow a light meter.
If you want to buy a 150FC spot simulate it be pointing a couple extra fixtures at the same area, put a meter on it and look at how bright 150FC is in your space compared to your normal wash.
The other important question to consider is color temperature. If you have a typical Source Four sort of wash, a follow spot in the 3000k range is going to look a lot like everything else. A follow spot in the 5000k range is going to pop even if it isn't that much brighter than the rest of the wash.
I second the color temp comment. I often use a RoscoI-cue dropped in a Source Four but gelled R60 or R61 and it will 'cut' through a front light wash of R02 or R07 fairly well. Most lower cost follow spots just don't have enough output. The old Altman 1000q type fixtures in my opinion were pretty much useless for most applications but I still see them at several schools around the area.
To add to @gafftaper 's comments - you will need to look at discharge or short arc lamp fixtures to get the 5000 kelvin range plus their output is generally higher as well.