Here are some comparison pics that I did for my blog awhile back of the available 15/18mm figures for the LoA. All shots are against a 4mm square background with interim marks every 2mm. You will see that there is a wide variance between ranges.
Officers & Sergeants:
The Les Higgins 20mm Marlburians were added just to give today's gamers some perspective. These are "true 20mm" (soles of feet to the eyes, as collector's measured them) and were considered the "gold standard" back in the day. It really gives a very graphic example of "scale creep", some of Lancashire Games figures are taller and they are bulkier figures.
While I own at least one battalion of each maker shown, the bulk of my forces are drawn from Essex, Irregular and Donnington. They mix well if kept in separate units, and some can even be mixed - judiciously
- like Irregular's standing firing or standing loading with Essex kneeling firing and officers. I have also done some head swaps that have worked out well and added some additional "character". By the way, while the overall height of the Essex figures is obviously shorter then both Donnington and Irregular, the physical proportions actually match up reasonably well. I solved the height disparity by simply slipping a small round shim under each Essex figure. I punched these out of some thin plastic sheet with a 1/4" - punch. I don't have any pictures handy of my current LoA project, but here is a TYW group of French musketeers that mixes Museum, Donnington, Essex and Old Glory and I used the shims on these to get everything to mate up.
I don't have pics of the cavalry, but Essex offer the widest selection. Both of the Donnington and Irregular riders will fit well on Essex horses, plus Irregular makes some nice horses. Since all three makers will sell separate riders and horses, I chose to use primarily an assortment of Essex horses (just cause I like them) with riders from all three makers.
Also by the way, there is at least one more maker, albeit a very small range. Mick Yarrow of MY Miniatures has a small range for the Glorious Revolution that includes some unusual figures, like rebel musketeers and officers, that will mix with these as well. I did a small test shot of some of Mick's figures, unwashed unfortunately, with a Donnington officer and the Irregular Sergeant for comparison.
I think you could pull together a very decent force from these ranges that will reward good painting and do you proud. I hope this helps with your decision.