Ok so for gundam model kits there are grades or ranks for size and detail and price I'll make a list and tell you where you can start
here's the list
1. Perfect Grade(Really detailed and has engineering beyond it's time) - 1/60 scale_price 160/300$
2. Master Grade(Great Engineering for it's scale) - 1/100 scale_price - 30-80$
3. Real Grade(Really detailed for it's scale and simple to build with cool mechanics) - 1/144 scale_price - 20-50$
4. High Grade(Really Simple to build for its' scale) - 1/144 scale_price - 20- 30$
5. Entry Grade(For beginners) - 1/144 scale_price - 10-15$
I'd recommend if you're experienced you can get into these specific model kits you get a real grade or a high grade.
If you're into miniatures you can build gundam Artifacts there's a few series and they come with limited edition colors
The model kits I went with was the god gundam real grade it has great articulation and amazing detail when it comes to a real grade. The model kit I'd recommend is the real grade RX-78-2 ver 2.0.
And to buy gundam model kits you can try USAGUNDAM, GundamPlanet, BigBadToyStore, Amazon. But for amazon and bigbadtoystore they can get a bit pricey and sometimes they doesn't have some of the specific kits you want to get.