As mentioned, MAN-EWR is on a 757 narrowbody. Not the most spacious for an 8-9 hour flight but it does allow United (and American who fly a 757 to JFK) to operate a non-stop.
All United's international 757's have in seat video with full AVOD. There's a very good selection of movies, TV shows and documentaries, though the TV side is very much geared to the US passenger. I would also however take something to keep yourselves occupied as there's always the possibility that the odd in seat TV plays up and you could have the rotten luck of it being yours (that's the case for all airlines though).
The Economy Plus seating is extra legroom only; it's not a separate premium cabin like BA's WT+. But, that extra legroom is around 4-5", which is huge, especially for long flights. You mentioned something about "cheap" on another thread. Last time I checked the going rate for E+ was in the £70-80 range, per seat each way. With multiple people in your party that probably doesn't sound cheap, so it's your call whether to go for them. They are hugely popular, so if you do want them reserve them well in advance using the reservation system on united.com. Even if you choose not to, the regular economy seats have a 31" seat pitch which is the same as BA's economy.
Food is economy class food; definitely nothing to write home about and it could do with improvement, so you could take a snack bar or two to fill you up. United does charge for alcohol in economy, and payment is by plastic only. Best to use a **credit** card for this as we had a thread last week from someone who was charged a bank fee of £2 (if I recall correctly) for each purchase on his debit card.
I fly United a lot, and generally I find them to be a good airline, and improving. They merged with Continental, and a lot of the justified complaints last year were due to issues that came out of the full operational merger (slow and sometimes crashing reservation system, and lots of mechanical delays when they started to fly Continental aircraft into United hubs and vice versa, and the hubs didn't have enough maintenance resource for "the other airline's planes"). But those have both been addressed, and performance is good again, and imo the flight attendant service is generally excellent.