Welcome to Android Central! In general, I favor Chromebooks with an i3 or i5 processor. The ones with a Celeron or Mediatek CPU are ok for basic everyday usage, but if you do plan to do any moderately intensive productivity or gaming, those lower end CPUs might give you more lag. You need at least 4 GB of RAM, but that's not hard to find -- you mostly find 2 GB of RAM on really cheap or older Chromebooks. 8 GB of RAM is nice, but it's hard to find that in the $300-400 range.
The HP X360 Chromebook 14 is a great all-around machine that I use a lot at home. It has a Core i3, 8 GB of RAM, 64 GB SSD, nice backlit keyboard, decent touchscreen, nice speakers. It often goes on sale at Best Buy for $349, but is currently $499:
www.bestbuy.com/site/hp-2-in-1-14-t...mc-flash-memory-white/6365772.p?skuId=6365772. If you plan to wait a bit, then keep checking Best Buy to see if the price drops that low again.
I didn't mention the newer Chromebooks with AMD CPUs because I don't know enough of how they compare with the usual Intel or Mediatek CPUs. I'm guessing the A4 CPUs are relatively slow, but the A6 might be pretty good.