What is progressive scan?
In order to understand progressive scan, you have to understand how traditional older TV displays work. On a standard TV the display is made up of lines of picture elements (pixels, in computer-speak) that are drawn exceptionally fast -- 50 times per second on a 50Hz PAL display. Lines are drawn in an alternating fashion, so if a screen was made up of only ten lines, lines 1, 3, 5, 7 and 9 would be drawn in one pass, followed by lines 2,4,6, and 8. It's fast enough to be virtually imperceptible to the eye, but there is the possibility of some visual flicker, especially for horizontal lines or high definition video -- like that on a DVD. Progressive scan doesn't use alternating lines, instead drawing every line in a display every second; the end result is a cleaner image that's more pleasing to the eye. Many DVD Combo units (and many DVD players and standalone recorders) support progressive scan. It's worth noting that unless you've got a reasonably new TV of a decent size, you're unlikely to see the improvements that progressive scan offers.
VCR Heads: How many is enough?
The heads on a VCR are used to read and record video and audio to tape. A standard VCR, working at normal speeds (3hrs to a normal E180 tape) only needs two heads to perform this function, but many manufacturers offer VCRs with four heads instead. This is done for two reasons. Firstly, increasing the number of heads increases the reading accuracy of tapes, leading to smoother pictures and better slow and fast tracking. Secondly, the additional heads enable better recording at different speeds. If you've ever recorded using a VCR and selected "Long Play" (or similar) functions to fit six hours onto a three hour tape, you've done so via a four-head VCR. Given that one of the key appeal factors for a DVD Combo unit is in being able to transfer your tapes to discs, you're much better off with a four-head unit than a two-head one.
As with any standalone VCR or DVD Player, you need to connect up a DVD Combo drive to an external display and audio output before it's of any use to you at all. At a bare minimum, you should be looking at standard RF Coaxial inputs (for antennae) and outputs, although that'll give you the worst quality picture. Composite cables (often called RCA cables) will give you a clearer picture, as will S-Video, Output quality will largely depend on the nature of what you're hooking your DVD Combo unit up to, but there's one additional input that you should look into if you're keen on making home movies. Many DVD Recorders, and DVD Combo units include an IEEE 1394 interface -- often referred to as Firewire, or i.Link if you're Sony -- for easy connection to a digital video camera. Apart from providing an easy connection, IEEE 1394 is a purely digital interface, meaning that any video that you transfer to a DVD Combo unit will retain quality when burning to DVD.