You'll spend a fortune if you replace all the coils with new ones, and in reality there's no need to. What is far more economical is to rebuild what you have. Essentially this means replacing the internal capacitor (with a specific type), and replacing the points if they are in poor condition. The coils must then be adjusted correctly to set the correct current draw and eliminate double sparking (important if you run from magneto).
I wrote an article for the club magazine early this year on how to do this.
There are a lot of traps for the unwary which has resulted in frustration and a lot of T owners installing a conventional distributor ignition. But, if done correctly it is a simple methodical procedure. I know many like to take short cuts or substitute parts, but this is one area correct procedure has to to be followed.
There is much more than just making a coil buzz and produce a spark, but if done correctly, reliability and performance is as good as distributor ignition. I'm now up to 22,000km with mine and haven't had to touch a thing.
To answer your question though, I'll say something about the plastic coils first. The Model T coils lived on way beyond the era of the Model T itself...many farmers built electric fences with them, and they have been used for ignition in furnaces, propane heaters, stationary engines, etc. Once the coils were no longer produced for Ford cars, various clones were produced for these markets; some of these were plastic cased. This is what it appears the plastic coils of today are. Two things need to be noted about them. Firstly, it would be difficult to access the internal guts to repair them, but more importantly, they may not function properly on magneto as the Ford coils were designed to do.
The wooden coils however, are the real deal and are built to Ford specifications so suit both magneto and battery power. Keep in mind there are some repro wood coils with the wrong capacitor in them.
Bear in mind that any coil regardless of where it came from is likely to need setting up before use, unless the seller has stated it has been set up on a hand cranked coil tester.