Your post confuses me - you say your an ISP but you use cheap VPS, and upgrade to second hand ebay servers - for commercial hosting? and your extreme limited knowledge of servers would worry me too. Remind me to avoid your company. I've only used Dell (I stay clear of HP) but the drives fit into caddys and plug directly into the backplain which has SATA type connectors so no messy cables. The servers accept SATA, but for commercial hosting I would recommend SAS drives instead. The backplain is then connected to a controller card, of which Dell did several models, a standone and some RAID ones. How you configure the drives is down to you (and your choice of controller) Again for commercial hosting you probably want a RAID controller, then you can allocate the combined disk space as you wish. The Dell servers have limit of 2TB drives, so a 1950(1U) would give you a 4TB raid array (or 2TB if mirrored). I'd recommend moving to a 2U version such as 2950, so you can fit more drives in and create a RAID5 array which is more resliant against single drive failures and improves performance. EDIT: I see yours takes more than 2 drives, by using the 2.5" drive spec. Not a good idea as these drives are more expensive. Get a 2U server like 2950 that can take full size 3.5" drives instead.
Not commercial hosting! Its for our demo server, where we make the customer websites, then delete it after 1 week once the project is over.
And Im from India, here voltage input is 220-240v. Buying a server from USA, do I need to change something, as I know USA runs on 110-120v.
Can't you find some supplier more local? These things are heavy and would cost a fortune to ship from US.