I've never driven a 1st gen '90-'94 Legacy, but from what I understand they are extremely similar mechanically to the 2nd Gen '95-'99 Legacy, Body and interior wise there are pretty big changes between the two. I've owned a '95 Legacy for 7 years now and I have to say that I really like the car. It's simple and light and easy to work on. It doesn't have a lot of power but the low weight helps to make up for that.
It might help if you give an idea of what kind of roads/terrian and weather you're planning on seeing on your trip.
One thing that comes to mind is that 2nd gen legacies are OBDII, which can be more convenient when it comes to diagnostics.
If you're going for a manual you will probably see a lot of cars with the 2.2l EJ22, stay away from the Phase II EJ22 engine, it's not as reliable. I believe '98 was the first year for the Phase II, or maybe produced in '98 for the '99 model year. Someone will know the exact year.
Also, be aware of the EJ25D, from '96-'99 here in the U.S. It can be a good engine but only if the headgaskets have been changed by someone who knew what they were doing. Although this one is less common to see paired with a Manual except in a few models or if someone did a swap.
I'm sure there are others who will comment with much more info about the differences between the two generations that I can.
I don't know what used car prices for these models are like in your area, but I know up here in the PNW one can find a very nice 90's Legacy for $5k. I would watch out for anything with too low of miles though because any car that's sat around that much could have issues just from not being run or driven regularly; but it depends on how it was stored.
Good Luck with your search! It sounds like you're going to have quite an adventure.
If you have any specific questions about the 2nd Gen Legacy, I may be able to answer those. I've done an engine swap on mine and almost all of my own work on it except welding.