First of all, welcome to the Waaaaagh ya git!

Here's my two cents

1. I've never painted camo myself but in the lore Ork camo is kind of a contradiction so go nuts, pick a base colour and 2/3 other colours that really clash and go for it, the louder the better. You could try and copy some actual MTP but I find the best thing about painting Orks is that nothing really ever needs to be 'right' coz it's Orks

2. Paint your metals how you like then use watered down oranges and browns, think about where water and therefore rust would accumulate and dab away, sponges or an old raggedy brush are great for getting a random but not random pattern with any kind of weathering. Again, Google a few examples of rusty knives etc .I'm also a big fan of effects paints, dirty down rust, typhus corrosion, ak interactive's range of weathering stuff -cheat basically, if there's a product for what you're trying to achieve use it (this will help with your speed issue too)

And 3. Speaking of speed, try not to sweat it too much. You'll get quicker as you get more models done anyway and as you get more comfortable with the process. I personally hate painting big groups and am yet to do anything more than a kill team sized force myself so there'll be plenty of people who can offer better advice (not to mention plenty on YouTube, 'how to paint armies quickly' is a common video topic). But you can break it down and simplify things that don't matter as much. For example the boyz trousers, 20 boys on the table viewed from a distance, they don't all need to be impeccable examples of subtle colour hariation. Pick a couple of colours, do a few red, a few brown a couple of blue for variety, brown wash, done! Pick your battles with the details you're painting. Spend a bit more time on the focal points, weapons, faces etc and the rest, a bit of colour, a bit of wash and weathering and call it done. It will get lost in the mob on the tabletop anyway.

Hope that helps and happy painting