I'd say that Upminster is the best on your list. You won't get a massive house for your money but you'd get a decent semi. I think all the primary schools are fine. For secondary, Coopers is selective, you have to have a religious faith among other criteria, and Hall Mead is a regular state school that's extremely highly regarded and a big part of the reason Upminster is more expensive than surrounding areas. The C2C trains into Fenchurch Street are fast and reliable, you can also get to the coast (Southend, Leigh, Chalkwell) quickly. The town centre and Upminster Park are really nice. Upminster isn't particularly diverse but it's gradually becoming more so.
Hornchurch is very mixed in terms of property/affluence etc and a bit more diverse, the high street is nice but not as good as Upminster. It has several good schools, a theatre, an arts centre and a nice new leisure centre. Transport wise you've got Hornchurch Station on the District Line or the tiny Emerson Park station on the "push & pull" between Romford & Upminster.
When people think of Gidea Park they usually think of the Exhibition Estate conservation area - beautiful early 20th century detached houses surrounding Raphael Park and the golf course. It's really
gorgeous but you wouldn't get much for less than a million. The rest of the RM2 (GP) postcode is still mostly nice and much more affordable. Main Road is the main "centre" of GP and it's only a short stretch but does have a good deli, the lovely Ship Pub, a couple of decent Indian restaurants, etc. Gidea Park station is connected to Stratford and Liverpool St. But when the Elizabeth Line opens properly we'll be able to get a direct train to the West End or even Heathrow! There's also a David Lloyd, which gives you another option for swimming lessons. Much more pricey than the leisure centre in Hornchurch but it does have a creche, kids club, outdoor pool and loads of family activities.
Of the areas you've mentioned, Elm Park is the least desirable. It's fine, I know plenty of people raising families there quite happily, but it's a little more rough around the edges and I wouldn't consider it with your budget.
Enjoy your visit tomorrow and feel free to PM me if you want to know anything else.
By the way my lunch recommendation would be Xich Lo, a little Vietnamese cafe on Hornchurch High Street.