Personally I prefer IBM/Lenovo over Dell, HP machines. Just trouble free IMO but I guess you can buy any brand among these cause you'll only find from these brands here.
If you're not planning to go for discrete mid to high level GPU then a 300-watt PSU is more then enough and I am sure those chines made PSUs (but A grade quality) in those branded machines can even handle beginner to early mid level GPUs fairly easily. That Dell Quad-core system sound promising (its a a shame systems with nearing 3GHz or over Quad-cores and their extreme variants are not available easily here... they surely can still compete with most i-series processors of now days). Al thought the asking price is bit high though. Should be not more then 12K at most for those specs.
Yes these PCs are from 07 and/or 08... they simply get outdated/out of warranty and gets dumped here in fairly excellent conditions (most of em but that is not say you don't have to make sure).
For VMs, I guess Quad-core system should be good I guess...(never been into Virtual Machines to be honest). But C2D nearing 3GHz or over with 4Gig ram should be able to do the same also.
Personally, I have IBM/Lenovo 3.16GHz C2D E8400S, 6Gig Ram with Q45 chipset for a year now with no discrete GPU... and ~30GB remuxes runs just fine bought in 14K last without HDD (replaced 250GB HDD with 2gig ram making it 6gig while buying). Totally satisfied with it.
1. Yes they support sata
2. 8 and in some cases, 16GB depending on the chip-set (mine does 16gig being q45)
3. For that, you need to buy tower cased (they usually come in Mini and micro-ATX casing, go for the mini or larger so can fit in two or probably more HDDs). Don't buy desktop cased. they don't have room at all for a second HDD plus you won't be able fit in standard size discrete GPU or any other standard size PCI/PCIe peripheral of any sort for that matter i.e NICs with standard mounting bracket (al thought some cards (GPU/NIC) are avaible in the market with mini-brackets to counter this)
4. Chances are pretty strong that they must have at least 2 PCI slots. Mine has 2 PCI slots and I sometimes use the wireless PCI NIC on it... even if somehow the one you buy doesn't have a PCI slot, PCIe slot with a PCIe NIC can be your equal alternative.
Hooking them up with UPS totally depends upon your needs. I have mine on UPS and the machine consumes just 90-watts. and as per my experience with IBM systems, they can or rather their PSUs can handle power outages very well. they protect themselves and rest of the hardware 99.99% of the time.
I suggest you go to the market to browse for systems but don't forget to note the serial/model number of the ones you like... and then check for detailed original specs of said model before buying or you can ask for further suggestions here if needed.
Hope this helps