As it's only theoretical....
In 28mmm you're spoilt for choice, The Empress figures are lovely, & the TAG ones also look very nice. If only I had the budget. Sizewise I haven't seen them in the flesh. Redoubt are big figures, probably nearer 30mm & IMHO too big to go with the other ranges.
The Foundry range, although possibly a touch smaller, have some nice vignettes that the other companies don't make, they're Perry sculpts. The Perry's own figures match up well with them as do the Warlord plastic figures (& presumably the WL metal figures too, although I've only got the plastic ones). The Warlord plastics are good too, especially for rank and file figures, but then you need to assemble them which can be a pain.
Eureka have a nice Scots/Montrose Irish ECW range, compatible with Perry/Foundry/Warlord.
As to reading, it depends how detailed you want to be. The various Ospreys are useful primers but the uniform details in the older books (eg Peter Young's one) are somewhat dubious. The newer ones (By Keith Roberts IIRC who knows his stuff) are most useful. The old Hawthornwaite on the ECW is good, again except for uniform details. There are a number of decent recent general histories which cover most of the salient points.
A couple of handy websites:
http://www.battlefieldstrust.com/resour ... -entrance/ for information about many of the key battles of the war(s) (three ECW, and the two Bishop's Wars, and the Anglo-Irish conflict).
https://www.baccus6mm.com/PaintingGuides/ECW/ Pete Berry is an authority on the ECW & provides some useful basic uniform etc details.
Timmo is your man for the real nitty gritty of the uniforms etc.
Never sell the precious, which is why I have ECW in 1/300, 15mm and 28mm...
and already has a set of rules that he's intended to use for 15mm armies but found that he didn't have enough figures but still has some guilt about this because he or she already has lots of projects on the go, loves painting but hardly ever plays a game
