I've taught a few people at your level of experience; it may take a little longer than the course minimum but that's not unusual. If you're reasonably switched-on and still in learning mode you'll be fine. It may be cheaper overall if you convert onto something like an Arrow before starting the MEP course because you'll be familiar with gear and props (the same systems on an Arrow and a Seneca) but set against this you may be better doing the complex conversion in the MEP because you will get more handling time and will therefore be more comfortable in the twin - depends on depth of pocket, motivation, blahblah.
The organisations you mention are almost certainly OK but they use the BE76 and this is particularly easy to fly. Being easy to fly and handle when asymmetric is a good characteristic if you're doing your IR but not necessarily what you need for basic MEP training. You also need to consider how easy it will be to rent afterwards, when you want to build-up experience.
I do some of the MEP training from Old Sarum (near Salisbury) and we use a Seneca 1. The Seneca is more difficult to fly well than a BE76 and IMHO this makes it a much better basic MEP trainer. Providing you are current on type there's no difficulty renting after the course.
Regarding books ... useful but not necessary because you should be taught everything you need during the course.
HFD