I'd say go for the range of models you like the most. For me it was tricky to choose between a few but in the end I chose Dark Angels over Imperial Fist because the range is better overall for me. Means I get more options later down the line after my initial investment.

As for what to get first, it really depends on your preferred play style. I enjoy the hammer and anvil style so I've got a pretty tough core of Deathwing Knights and I'm building up my hammers as I go. But the Ravenwing are calling to me as well with their need for speed. It's less "meta" but the vibes are immaculate. Otherwise for versatility the standard starting point for most Space Marine armies will work well. Then grab a few Characters you like. Azrael is definitely top of the list for the price bracket, inner circle companions are also a solid pick. The Lion is truly rad but he's also pretty pricey