I use the cheapest Etrex model which doesn't have any mapping capability. You should be able to pick one up for about £60 now. I transfer the information from a route sheet to my Mapping software (Memory Map) which has OS 1:50,000 mapping for the entire UK and double check anything that I'm not certain about using Google Maps for road names.
Here's some info. cobbled together from various things I've written before about audax navigation using the Etrex:
This model has two navigation modes. One displays a direction arrow and distance to next waypoint. I quickly realised that I didn't like that mode. It doesn't give any indication of what is coming up until you are almost upon it and then there can be ambiguity about where to go next. I tried it and a couple of times I found the arrow pointing the wrong way at junctions and then correcting itself after I'd turned - useless!
I prefer to navigate using the 'breadcrumb trail' mode. This displays the shape of the route ahead as a line on the screen with an icon of a little man representing my position. I set the device to auto-rotate the image so that I am
always riding 'up' the screen (otherwise it would show North at the top of the screen and I'd have to effectively rotate the image mentally to work out what to do next).
I usually have the scale set to about 150 metres per cm so I can see what is coming about 750 m ahead. That gives me plenty of time to think about position on the road and signalling. It also lets me spot any complicated navigational issues which may arise so they don't catch me out at the last minute. If there is a right turn closely followed by a left, I can see that and am ready for it.
The device uses two AA cells which is very convenient if you ever need to buy replacements out on the road. It drains about 100 mA when not using the backlight. I never use the backlight because I rarely ride at night. On the few occasions that I have, I've used a head torch to see the display. I have some 2,600 mAH NiMH cells and can confirm that I get over 24 hours continuous use out of them at normal tempertures so that would be enough for a 300 or a moderately quick 400. Two pairs of cells should see you through a 600. Note, as always - battery life may be impaired in very cold conditions but you probably don't do very long rides in the winter (do you?).
My Etrex has been ultra reliable but I always carry the route sheet and a map for backup. I've only had two problems in about 3,000 kms of riding with the device.
Problem one was when I used some 2,000 mAH cells. They turned out to be slightly physically smaller than the ones I use now. As a result, they were not a tight fit in the battery compartment and tended to rattle when doing fast descents on rough road surfaces. The GPS cut out a couple of times before I worked out what the problem was. Use larger cells or pad out the battery compartment with a strip of paper to take up any slack.
Problem two is missing turns when I don't look at the screen often enough. The Etrex alerts the user that a turn is coming up by flashing a warning on the screen - it is no use whatsoever! If I'm already looking at the screen, I can see that without the warning. If I'm not, I don't see the warning either! On this model, there is no audible alert. I'd like to be able to set it to emit a loud 'beep' 100 metres before a turn. I've navigated over 3,000 km using my Etrex and have only missed 3 or 4 turns when talking to other riders. One glance at the screen tells me that I've gone astray and it is easy to get back on the right route.
Highly recommended! (Mind you, I know people who have the more expensive models with maps built in and they look great if you can afford them.)
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), as well as the Etrex Vista (likewise), which is currently heading my "can I justify this ?" list 
