assuming it is level ground then you want to be using 10 feet posts as a minimum.
Its fairly easy if you are used to doing it. Dont use metposts, concrete the posts in properly. I would personally always recomend closeboard fencing to my customers as it is much stronger and lasts much longer. It is that much more expensive to install either, But if you are a novice then it is easier to install panels.
If its panels then put in the post at each end and get it levelled up. run a string line between the two. Level up teh ground for the gravel board, and then mark the location of the next hole. Dig that. Fix the board and fence to the first post and then put the next post in against taht. You can now level it up and get it plumbed against the string line. Concrete it in position with a dry mix of concrete and ram it well in. Repeat all teh way along. at the end pour water onto the concrete.
That is basically it, but you may have some variances such as a sloping site or need for half panels etc.
you can buy closeboard panels from fencing suppliers, much stronger than lap panels.
The higher posts are becasue they need to be buried in the ground. rule of thumb is 1/3 of the height above ground (ie 8 feet) determines the depth of posts foundation, so you really need a nother 2 1/2 feet, but the trellis will not take a lot wind loading so you may get away with 2 feet.
Cheers, this is why I sometimes like to do jobs myself. I had a quoute where one guy stated that 9ft posts would be ok! Do you have a website or anything? Not sure what part of UK yr in as I may have a go if I have the time, but will get in a pro if need be.