I had separate ff and chart units on my boat. Because of the limited visability in the cockpit of an angler, I later opted to go with a combined unit to unclutter the dash. Then I had a problem with the chart plotter portion and learned the obvious benefit of having separate units.(I was then down both for almost a month while it was fixed). If you don't need a fancy ff there are hundreds of used options out there cheap.
When choosing your new chart unit, whatever you choose, keep in mind the types of charts it will take, their cost, and if you have some now, the cost of replacement if you go to another chip format. Also consider whether or not you might ever want to add radar, etc. What a "racket" those chart companies have (expensive, lousy service, no warranties, no exchanges if you relocate, poor upgrade programs like 75% to upgrade each year, poor web sites and almost no portability to new units. Then they pretty much limit the market for your used unit to only the local market).
With Raymarine only the E systems will take the new fancy Platinum CF charts. Certain others will take NT+ but not the newer MAX SD formats. The Ray E's are nice, but not inexpensive even with rebates. If you buy a used unit, make sure you get an original proof of purchase from the seller if it is still in the warranty period as it is needed by Ray, and probably others, to process under the standard warranty.
You must have a large boat if you can conveniently fit a 12. I'm jealous.
Chris