Mr Longbyt has a Garmin 400T. He says that the 550 is for roads - but 550T Topo seems to be OK - but EXPENSIVE. He suggests you buy from a discounter, which saves a lot of money. As you say, the maps are expensive too - but with the Topo they are included in the price.
Seems with the 550T you can 'save' more 'Waypoints' than with the 400T but are you going to need to?

I have a second-hand 'no-longer produced' model a Geko 101. It doesn't find the satellites as well in deep narrow valleys as Mr L's but it's fine for my needs. It gives me something to play with and keeps me out of mischief while Mr L takes photographs!