To upgrade to a color chart plotter with radar will cost a bundle of bucks. I would go with one of the small simple units. I have used Hummingbird, Eagle, but the most recent one I have been using on the Caracal 18 is a Lowrance X4 Pro (this model has the dual frequency transducer, and will give a better signal). This should be some where between $80 and $100.
It is grey scale/black and white, and only 4.25" wide and 5.75" high.
If you want to go to an upgrade (and I would start with a unit you can add onto--something like the Garmin 541S for about $600,. The 740S will run you over $1500, plus $1100 for the Radar. Once you split a 7" screen, you will find you are better off with a 5" screen for the chart plotter and a separate radar and depth sounder.
I would stick with the monochrome Furuno Radar, which is a good unit, add the simple fish finder, and when you have the $$ add something like the 541 or 541S or 7" if you can afford it. (Both of these Garmin have all of the US coastal charts built in (or other models have the lake and inland waters--chips will give you the inland waters in the coastal models). I personally like at least a 5" chart plotter. You can buy 3" or 4" for slightly less, but I think that 5" is a minim for seeing the chart clearly when you are running at planing speed. Keep the Furuno GPS--as a back up, but move it down below or to the other side--and use it for way points and cross track error. I often run two GPS plotters.