I personally wouldn't use one full time. The mattresses are not good for their spine, even the foam ones you can buy for them aren't good for long term use. Also, like you said, the constant bending over puts strain on your own back. If you have to settle your bub off to sleep by patting them it would be hard work!

Have you considered buying a second hand cot and just buying a new mattress? You can buy cheap second hand cots from eBay. Just make sure the cot isn't too old and meets the current safety standards (you can Google the standards if you want to know the specifics).

A portacot would be ideal for your beach house though!