Apple TV HD or 4K

A 4K Apple TV works just fine with a non-4K TV assuming the TV has HDMI inputs. You are correct, getting the 4K version is a better option in the long term. When I got my 4K ATV, I still had a non 4K TV and it worked fine. When I got my 4K TV, I simply plugged it in and began enjoying it again. If there was Apple TV 4K content that needed a subscription (like Netflix) I just upgraded my subscription and began enjoying. If you get the 4K, also buy a high quality HDMI cable so you don't have to spend more money later when you buy a 4K TV. One like https://www.bestbuy.com/site/rocketfish-4-4k-ultrahd-hdr-in-wall-rated-hdmi-cable-black/3720002.p?skuId=3720002 is fine.

One last thing, also when you get your 4K TV ensure your network at home is robust as 4K takes more bandwidth. If you have a large home (1500 sq. ft or larger) and/or support a lot of wi-fi devices at home (over 5-10) then you should look into a high speed internet connection AND a Mesh Networking system like:

https://www.bhphotovideo.com/c/product/1289768-REG/google_ga3a00441_a14_z05_google_wi_fi_3_pack.html/?ap=y&ap=y&smp=y&smp=y&lsft=BI%3A514&gclid=Cj0KCQjw09HzBRDrARIsAG60GP8H01b_W0Zc6bskj1tTyraMEVg3XsTQu1NutEr8o-JU6pKFeEd-Q1waAlt1EALw_wcB

I had no reason to select this particular vendor, you can do your own shopping when/if the time comes. Same holds true for the HDMI cable.