Just purchased a new to me 1998 Champion 181 and need to replace the outdated electronics. I am on a budget so trying to spend wisely. Is GPS a helpful feature (presumably to follow contours) for the FRONT unit? Is down imaging a useful feature?

Yes on both questions.

I would recommend with going at least a 9" if you do put one up there... It will help to see easier esp if you start splitting the screens for different views.
Good luck with your search.

Im cheap and put a lowrance hook 5 up front on trolling motor.

Have a Garmin 7DV up front that has 2D chirp and down imaging. The down imaging is really nice, especially in submerged timber. Also has the ability to do way points, but no mapping. With the waypoint capability, I haven't had a need for mapping yet. Get close with the mapping at the console.

You will want your front unit to do as much as possible.
In order of most importance to me.
- Being able to see the map and follow the contour line out and around points, humps and creek channel swings.
- Having Down Scan is much more vivid a picture than sonar. I want that image of the tree, bush, edge of grass , ledge and the fish relating to those features.