Bubblegum, if you do get one of the cheap coldframes, don't bother putting the lid lifting thing on, it's a waste of time. I'd also put it in a sheltered spot, and as it doesn't have a floor, save the cardboard it comes in and use that as a base, cut it to size and plonk it in the bottom, it will give some protection from ground frosts. Don't just put the frame on top of them, the wind is likely to get underneath and blow the whole thing over. If you're in a very windy area, then a housebrick plonked on top of the roof will help stop it getting blown about, as it's a polycarbonate one (the one I bought was, anyway), so it doesn't weigh very much. I'd do that, rather than risk your pvc thing getting blown away with all your seedlings in. Failing that, can you not create some type of tie-down, using rope or washing line cord? You could always put some duck tape on the bottom, use a hole-punch to put holes in and then use pegs for weed-surpressing mats to hold the entire thing down.
Personally, having one I'd go for the polycarbonate coldframe every time, as it's lower it's less likely to get blown about, and you *should* be able to get all of the plug plants in. Get some cheap horticultural fleece if it gets really cold (poundstretcher or wilkos is a good, cheap solution). Acclimatise in march/april by leaving the lid open (shut it if frost is forecast), last couple of weeks remove from the frame completely (again, pop them back in if frost is forecast), then plant them out into their final positions. Keep the fleece handy in case of a late frost, and once they're planted out, chuck that over them if frost is forecast again. Next year they'll be much bigger and you shouldn't have to worry about them, they should have established a decent root system to survive all but the harshest winters.