Price is £2770 for a p18 with a 180kg weight with a bracket to suit the machine.
On one hand that sounds reasonable but Jesus it's a lot of money for something that really is quite simple. I went to bid on a used p18 at a local auction and bidding started at my max bid, and finished at something like £2200+vat. That was a good few years ago now, ended up making my own and it cost less than a grand in parts and steel but took plenty of evenings in the workshop to build.
180kg will be absolutely fine and should deal with decent sized strainers. I've put in massive oak strainers and if it's hard ground I have augered 6" holes first. Rock spike is great, helps posts go in on the line and not slip sideways. Making holes with a rock spike also means you can easily jab a load of stakes in and they'll stay upright while you're back in the digger. I usually jab in 10 posts or so and then knock them in, way faster than a tractor.