The RP2+ / RP3 / RP3+, Anbernic RG505, and Odroid Go Ultra are the only sub-$200 handhelds I know of that will comfortably play N64.
The normal RP3 looks like it's in stock if you want to save $30 and don't need the extra performance of the RP3+.
You might also consider a used/cheap Android phone with a good chipset. For example, Amazon has a renewed Moto G Power (Snapdragon 662) for $110. Add a grip like this for less than $20 and you're set.
https://www.amazon.com/gp/product/B0BHHFZZFX/ref=ppx_yo_dt_b_asin_title_o05_s00?ie=UTF8&psc=1
This case is out in a few days. Looks good, ordered and waiting!
This is the best and most accessible version. It is the only game I play on my phone because I can look up at any time.
The digimonx pets are reasonably priced and pretty fun: https://www.amazon.com/Digimon-41921NP-DigimonX-Virtual-Tamagotchi/dp/B099CQ13MB/ref=sr_1_5?crid=93NA4MZ211SE&keywords=Digimon+pet&qid=1665271603&qu=eyJxc2MiOiIyLjMxIiwicXNhIjoiMi4wMyIsInFzcCI6IjEuMzUifQ%3D%3D&sprefix=digimon+pet%2... But I have been really enjoying the vital bracelet which is like a fitbit and digivice combined. You have to buy cards for it though so it is a lot more money in the long run.
It's this, pretty pricey but it's great: https://www.amazon.com/dp/B08H5HRB69
I'm going to be making a final post on the setup once I finish it tomorrow on r/steamdeck which will show now pictures and stuff