Why would you have the head unit/screen (the colour double din one?) and no navigation?? What do you use it for watching DVD's when stationery?
Are you sure you don't have a nav computer in the boot under the CD changer?
As to what is better I suppose its how much value you place on OEM and whether or not you are going to use any of the features of the Dynavin. In terms of cost I guess it will be about the same I have seen nav computers for sale on ebay for about $800 and GPS antenna for about $60.
Do you have factory bluetooth and PDC? If you do the Dynavin is not going to add much value feature wise. I would assume that OEM navigation would add greater resale value than an aftermarket one. So although the installation will be much more difficult (look up threads its quite involved seats, console and carpet have to come out) it may be worth it since you already have the head unit in place.
As to fitting since you already have the screen if you take that out you will have a double din slot available so yes the Dynavin will fit into that space as described.
Goodluck and let us know what you do.