The picture shows some sample UPSs and an example of the gel cell from one of them. The UPSs come in various capacities and, although you can boost the capacity, the output power is fixed. When you start out, make sure that the UPS you're going to modify will provide the volt-amps and power that you need. Also note that the volt-amp rating is higher than the power rating. The difference is because AC powered devices have a power factor. Check online for more info about this. Another, similar Instructable, also warns against trying to max out the capacity of your UPS because some use transformers that will run continuously at the rated output. It really depends upon the quality of the UPS, but plan to run at not more than about 75% of rated output capacity.
Another thing to consider is whether or not your scrap UPS has AVR or Automatic Voltage Regulation. You'll want this if you can find it.