Stick to standard on all the drive train, metric is fine for body fixings. I use Screwfix as a supplier of metric fixings, much cheaper than old UNF stuff.
There's quite a few people sell nut and bolt sets in NF on ebay, worth getting on the shelf. I use the odd metric on the bodywork when in a hurry but I prefer to keep everything that should be imperial, imperial - leads to less confusion in the long run.