Honestly, since you are still trying to "find your way" I would recommend NOT buying ANYTHING yet. Instead, search these and other forum for different styles and types of bikes. Make notes of what you like and where to buy those parts. Try to picture in your head what each part would look like on your bike, and what each part would look like when used in conjunction with another part you also like. Do these parts work well together (stylistically)? Will they function as you need? Does it fit within your budget, or do you need to look for more affordable options (used perhaps)?
Just some things to consider BEFORE you spend your $$$. The more you figure out ahead of time the better off your project and wallet will be in the end.
Good luck!
What he said. Seriously.
Don't buy anything until you know 95% the direction you want to go and how you want the bike to turn out. It's much better to have a plan and work towards it, than buy a bunch of parts and change your plan and have to sell a bunch of brand new parts that people will only give you 50-75% of what they cost.
You also need to research to determine your budget. Whatever your budget is, add 50%. When you start rebuilding or piecing things back together you'll realize it's best to replace things when you have then apart....like forks, brakes, wheel bearings, carb boots, misc hardware.
You can buy cafe seats, from Ian Halcott(contact CB750 Cafe Racer Fan here on the forum for his info) Dime City, Harisuluv (member here), motorcycle seats direct, Kott Motorcycles, or make your own.
I got my parts from....here the forum, cognito moto, lossa engineering, vintagecb750, z1parts.net, and a ton of parts from your local honda dealer.