I'd be wary about opening the stern up for self-bailing on a boat going offshore. I see your reasoning....a lot of racing dinghies have this same design. But in offshore following seas, I wouldn't really want an open stern, although I think there HAVE been some ocean racers that have had open sterns. But I've never seen those type of wide, high-initial stability racing boats as really "safe" boats, beyond the benefit of "outracing" a storm front.
However, if you are good with fiberglass work, you can do things like build your own bridge-decks, and change out large portlights for smaller, more seaworthy portlights. Epoxy resin is good for stuff like that...it sticks to anything.
After you buy your low-cost boat this summer, and spend the summer learning on it, you will have a better idea if the boat, with modifications, would be good for such a trip. And like someone said, if you deem it NOT, you can always sell it and get something more substantial. I don't know enough about any of those three boats to specifically help you decide.
Looks like you already know the most important thing: to stop dreaming and start DOING!! That motivation will take you a long way!!! Good luck and fair winds.
: I just looked at some pix of the Challenger, and it DOES have a bridge deck. For it's going price (between $2500 and $3500), it doesn't seem a bad boat. I don't see the underbody. Skeg rudder or unprotected? The one I saw pix of had a decent sized interior if you live frugally and simply (a great thing to do anyway!!), although I would be wary of having a dinette instead of a true sea-berth. At least one good sea-berth on any passage-maker is VERY important for sleeping comfort and thus eventual safety. Besides that, it doesn't seem like a bad boat.
If the boats your talking about are on THIS Quebec site: Voiliers à vendre - Sailboats for sale
, I'd be VERY wary of their prices. The prices I've seen seem VERY high, even given the exchange rate. You should be able to find much better deals than these prices for these same boats, I would think. They have a 21 foot Challenger going for $7500 Cdn. That seems HIGH to me, and a Challenger 7.4 going for $9500 Cdn.....versus $2500 - $3500 US on sites like "SailingTexas.com"