Personally I'd see if your DVD player can upscale, and if it can output this over component cables you will be ok. If it can not I'd be tempted to buy a new DVD player that has upscaling built in and hdmi output. They are around $50 - $150 depending on the brand and if you buy them at the supermarket or not etc.
With regards the freeview HD, the 3 muxes are on different frequencies. In some cases your existing aerial will be good at picking up some of the muxes and not so good at the others. This can mean the TVNZ channels for instance might be fine, but a group of others might not be very good.
With freeview you either get it well or you have problems with reception. If reception is not so great then you will need to look at your aerial, and how it connects back to your TV. RG6 refers to the type of cable that has sheilding appropriate to the signals you are wanting to send down it. RG59 looks similar and is cheaper but does not offer the right protection for TV signals.
If you have other TV's without freeview built in or a DVD/HDD recorder/VHS then you will need to keep the VHF aerial diplexed in with the UHF aerial. If you only have an aerial to feed the TV then you should throw away the VHF aerial and cable the UHF aerial directly to your TV.
Address these issues and you should be happy with your purchase.