I personally find Budgies to be extremely social birds and do best with another bird in sight. Not necessarily "together", but to be able to see another bird around.
I would buy as big of a cage as you can afford. I have this cage and my Budgie uses EVERY INCH of it.

Budgies are EXTREMELY active, as in, give him a LOT of toys to play with. By this, I don't mean only bells and plastics you find in petstore. Give him soft wood such as Balsa, Yucca, and Palm to chew on. Budgies naturally need to shred material to keep down their beaks.
Yucca:

I would also offer foraging toys such as this. Fill it up with millet, seeds and watch your bird work for his food!

Here, these are photos of my cage set-up, toys, and easy homemade foraging devices. Take a look here->
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