You already know the importance of a strong foundation, so yeah chassis must be first. Safety features, steering assembly, tethered kill switch and good brakes top the list of firsts. Remember stock lawn mowers are designed to go 6 miles per hour. Stock steering and brakes just aren't enough, even if you only increase top speed to 15 MPH with a slight change in pulleys STOCK PARTS are just dangerous at any more than factory set speeds. Welding and parts fabrication that you can do yourself will obviously save money. Find a local club and go visit with them. Take your camera and take lots of pics.
Don't get overwhelmed, building a great machine takes an investment of both time and money. Study the ones that went before you and listen to sage advice, don't try to re-invent the wheel and you'll spend less in the long run.
Welcome.... from Afghanistan.