That EBay brooder might be OK for a little while but like you, I would be cautious about ventilation, and also there is no way to adjust the heat output as they feather up and need cooler temperatures in preparation for life outside. If they get hot they need to have plenty of room to move away fom the heat lamp. How many are you expecting to hatch?
A better bet would be a big deep box (I have used a cardboard box from a new microwave oven) with a heat lamp suspended over it, which you can raise or lower depending on what they need. You need to fix a mesh lid to keep the chicks in and the cats out (though it would be best to keep the chicks in a room the cats have no access to.) You could use a guinea pig cage, the kind with a plastic tray and a mesh top, which is easy to clean and portable. Maybe you could get a second hand one.
Alternatively, you could use a Brinsa Ecoglow electric hen. See
http://www.amazon.co.uk/Brinsea-Eco-Glow-Chick-Brooder/dp/B008HVM56Q/ref=sr_1_3?ie=UTF8&qid=1401217797&sr=8-3&keywords=chick+brooder
Which has adjustable sides so you can raise the heat pad as the chicks grow,. The power consumption is very low and they are useful. (PM me if you are interested in this option, as I have one spare, used but in perfect working order, which I could sell you for £15, half the advertisd price of a new one, with a free chick feeder and drinker thrown in. I used this for hatching quails but I haven't got them any more and have no plans to breed more chicks.)
By the time they are about 6weeks old they will do best at this time of year in a covered outside run with a little coop, on clean grass if possible. So brooder is only temporary and you need to wean them off heat as soon as you safely can. This is the ideal time of year to do this.
