First, welcome to the forum

Heritage of Scotland and John Morrison are owned by the same people, the Gold Brothers. Whilst they supply some good quality stuff they are also known for supplying the lower end of the market and you may want to research their reputation before using them

Any of the four tartan mills you mentioned are good and any kiltmaker should be able to provide you with colour swatches so you can see the colours and feel the weight, and my advice would be to look for a kiltmaker close to home. It is invariably cheaper because of import taxes etc and easier to deal with if anything goes wrong. I believe this forum owner is in Canada so that might be the obvious first choice for you (Freedom Kilts). Failing that in the US you have USA Kilts and the Scottish Tartans Museum shop. My fourth choice would be Scotweb which, although in the UK, are used to shipping your way. All of these retailers can provide everything you are likely to need and you can be assured of the quality

I'm in the UK and I would always recommend 16oz cloth. Wool breathes so is theoretically cool in summer and warm in winter. It has a better hang and swing than 13oz. But there is not a huge difference in weight and a casual observer will not be able to see any difference. I would simply go with whatever weight suits you best for colour, feel and sett size. Get samples sent to you and take some time considering which you like best

When you decide where you want to purchase from ask for advice from that retailer re the accessories. They all have access to the same suppliers and will guide you well. If they don't stock precisely what you want they will know where to direct you