Hi there. Never heard of a mid-strength porter, but at a glance I'd say use a Coopers Draught kit with 250g LDME, with a mini-mash of 300g Crystal and 100-150g Chocolate grains. Hop with any British hop - my choice would be Goldings but its up to you. 10g boiled for 15min with the grain liquor (after straining grains out) with another 10 either at flameout or in secondary will give you some nice character without overpowering a midstrength brew.
Ferment with S-04 or your choice of liquid yeast at 18-20'C and you're set.
Cheers - boingk