Trust me. Iv`e been in the business for 35 years. There�s no shortcut. Cheap is cheap at the end of the day. If you have a large area to cover go and hire a spray. Tell the chap exactly what you aim to do and they will give you a spray to suit the job. I use an airless spray, easy to use and easy to clean. Don�t use a spray anywhere near vehicles or anywhere that may gat wind blown vapour, i.e. washing on a line etc. Be warned professional sprays do a fantastic job, but a like a racing cars compared to the things from diy shops.
If you fancy doing it with a brush, don�t buy synthetic. They are rubbish. Try and get a proper hair brush.
Best of luck.