Tim,
Yes, USB 3 ports can read/write USB 2 disks at USB 2 speeds.
Large drives are reliable. Just pick the right model.
Regarding speed, I use SSDs where speed is needed. The speed difference between a 5400 RPM magnetic/conventional hard drive and a 7200 RPM drive is negligible compared to how even a slow SSD will perform. The 7200 is likely louder and hotter, and costs more than the 5400. For magnetic hard drives, I’d stick with a decent 5400 drive. Many good options exist. Samsung/Seagate STxx00 series for the 2.5" format, Western Digital Red EFRX for 3.5".
What is “large” to you, btw? 2TB? 6TB or more?
On a general note, I prefer buying enclosure and disk separately. This is not the cheapest option, but you can pick the best performing parts, and you will have a repair and upgrade path as well. Assembly is easy. I can recommend specific parts if you are interested.