We use a Prime chiller for both our 140 (model 2646 1/3 hp) and our 45 (1/10 hp). The smaller unit chills our Abdominalis tank to 68 and the larger keeps our reef at 72. We don't feel we are pushing either of these units and could easily make them cooler. We have hade the 1/10 for about 2 years and the 1/10 over a year without any problems (we bought the same brand for the 140 because we liked the first one so well). A comparison of the different Prime chillers and example pricing can be found here:
http://www.marineandreef.com/shoppro/chillers_current.html
I am not advocating marine and reef but we did get the second one here because of pricing and a discount that they offered at the time and have
no complaints. We do not know why the Prime/Current brand is considerably less expensive than some of the others

but they are relatively quiet and have been problem free. We do hear the 1/4 hp unit but not the 1/10.
If you find a smaller used chiller (1/10)
really (read that cheap!) reasonable - PM me!!!! - since it would be too small for your tank but ideal for ours! So far cruising the "Bay", we see chillers selling for almost retail.
We would like to acquire one of Zy's critters even at this age but won't without a chiller and just can't afford another new one right now. The sump for our octo tank is enclosed and the best I can do with two fans is maintain ambient (75 ish) so a chiller is required (keeping the house at 70 would be both uncomfortable and cost as much as a chiller in one summer!). Additionally, after using fans and chillers over time, I vote for chillers whenever you CAN use/afford them. I still use fans on several tanks but would love not to.