Duncan
An ordinary gps will do the job of navigation but you have to input all the waypoints/marks yourself. Most of us used to run on gps alone as plotters were not around then, and to be fair it worked fine. When plotters hit the market I thought no way am I going to pay for one of those new fangled things but peer pressure finally made me give in and I got the latest must have toy. WOW what a difference. Its just like having an electronic chart there in front of you. You can look at the chart put your cursor on a fishy looking bit of ground and go to it. If the fog comes in you can easily navigate your way back home whether you have input waypoints or not, cause you you have the chart there in front of you.
Regarding price, it depends on your budget all I will say is that colour is better than monochrome and you should try and buy as big a screen as you can afford. On the smaller screens your unit will need to rewrite more frequently.
Regarding anchors most of us around this way use a Bruce style. These are very good for holding in all types of bottom except for very rough ground, a 5 kilo Bruce would suit your boat. For very rough ground a fishermans or grapnell would be best. You should have at least one boat length of suitable chain 6 metres for your boat would be fine and at least 3 times the depth you intend to fish. I would suggest that 10mm warp would suit you or maximum 12mm thick. Bear in mind the more rope you buy the cheaper it becomes, I usually buy a whole coil which is 220 metres. If you source it from a supplier that serves commercial fishermen you should be able to buy it very cheaply. An Alderney ring (available mail order)and 30/35cm bouy would complete your anchor needs.
Martin