I am the last person who should be giving advice on GPS - given I bought my Etrex 1 week before I went on my trip and have fumbled my way through the basics of using it since then - so take anything I say with care. You've been warned!
Etrex - come in lots of models and capabilities. The most basic will only give you your location and a bearing / distance to a set way point, the more capable (expensive) models will allow you to install maps and "auto-route" to set destinations just like your normal in-car navigators. To install maps you need one of the Etrex models which will accept a microSD card - which I think are the Etrex Vista HCx and Etrex Legend HCx (I have a Legend HCx).
I've been pleased with my Etrex - it has worked well since I learnt to put maps on it (which wasn't intuitive) and which buttons to push to set waypoints etc (which also are not always intuitive). I know it is possible to set routes up on it - but I use it in combination with a map and simply select next town I want to get to in turn.
Maps - well you can buy them from Garmin of course but there are free maps known as OSM available on the web. I found most of the information about these totally unintelligible and it really put me off them at first. However I then found the website Free worldwide routable Garmin maps from OpenStreetMap
where the host has done most of the work and you can down load maps by country ready to use. To put them onto the Etrex you have to copy them onto the microSD card and this is where Garmin makes it harder than it should be. You have to create a folder "Garmin" and then store your newly acquired map in this folder with the name "gmapsupp" - nothing else will do (you can't put two maps on and call one India and the other Pakistan - it is strictly one at a time). Consequently if you are going to more than one country you need to have several microSD cards prepared or a laptop do it on the road.
Overall I've found the usability of the OSM maps great and the level of detail superb. Having initially been a GPS doubter I'm now a convert - when coming into a strange town late in the day and when you are tired it takes a lot of stress away knowing you are being routed to the centre or to your hotel.
So in summary my advice would be (a) make sure any GPS you buy has the capability to install maps and (b) buy as early as possible and use it around your home area so at least you have a feel for it before you go.
Good luck - hope this helps a little.