Heated propagators

Heated propagators

I have a question. I want to germinate the seed of sub-tropical plants. Last year I had a few that didn't work out for me, so it's time for a heated propagator. However, I don't want to pay out for one with thermostatic control and sellers give no idea of what temperature they are able to maintain and in what ambient temperature. Can I get away with an eight watt model, which seems the cheapest, or am I likely to need something with more wattage to keep the temperature at what's needed, which I guess is about 25C/77F for, say, tender passiflora species? I'm zone 8, have a (crowded) windowsill indoors and a frost-free greenhouse - i'd rather use the latter. Thanks for your help.
Some pictures of my garden

Some pictures of my garden

bags packed, we’re on a plane to heaven.

bags packed, we’re on a plane to heaven.

sad plants

sad plants

Today’s harvest.

Today’s harvest.

everlasting

everlasting

Don’t you love spring?

Don’t you love spring?