Any HD (nearly) for T320 w/ H710 good?

"Can I use any SAS or SATA drive without any issues? I mean they should be good enough to be used in RAID etc. but i mean is the Dell gonna wanna see Dell only drives? The reason I'm asking is that I may already have some drives in house, or want to add them later. "

You can use any SAS or SATA drives, but which ones you use can determine whether or not you will have issues.  DON'T use consumer/desktop/laptop drives.  Enterprise class drives should be used at a minimum.  Certified drives are recommended for maximum reliability or if you will ever require Dell Support to assist in troubleshooting storage-related issues.  Generic/retail non-certified enterprise drives can be used - usually without issue - as long as your support team is adept at troubleshooting storage/RAID issues.  Some generic/retail drives have been tested by the drive manufacturers to be fully compatible (like WD RE4's), although they are not considered "certified" or guaranteed by Dell.  If shopping for Dell-certified drives, you do NOT have to pay the high price of buying directly from Dell ... they can be purchased from a number of online retailers/suppliers.

"Also, since I'm only going to order 2 SAS drives to come with it, are all the other empty bays going to come with hot swap HD trays so that I can mount whatever HD into them later:?"

No, unconfigured drive bays will come with blanks, not empty carriers/caddies, so you will need to purchase the carriers from suppliers as well (or with the drives).

3.5":  F238F, X968D, G302D
2.5":  G176J, Y961D, G281D

There may be others, but that should get you started.

Many Dell-certified drives you purchase from suppliers will include the carriers.