Largely an issue of somatics for what you are talking about.

When I talk spray guns - I am thinking of painting a car or a don't want that. What you are probably after is a cheap, single action airbrush. For that task, I have been using a Badger 350 for decades (think mine will be turning 24 or 25 this August). It is a simple brush which you should be able to pick up for cheap (I have seen them for less than $30 in the US - with knock offs going for less than $20). You can spray pretty much anything through it from inks to house paint. Cleaning takes about half a minute. Head over to the Badger site and then compare what you are looking at to one of those. Several of the knock offs are as good as the Badger, but do end up costing less. Some of them though use cheap plastic nozzles which end up clogging or having manufacturing issues that interfere with the paint/air flow.

The issue you will likely run into though is that probably as important as the brush is the compressor. Canned air isn't a very good option. You can use larger storage tanks if you like though, the regulators add to the cost benefit (they end up being nice since they are quiet though). If you get a cheap compressor that pulses - your paint will pulse too...and that is a bad thing. Spend a bit on a good compressor or a pressure tank and regulator setup (you can fill the tanks at your local gas/service station or head over to a specialist who can fill it with nitrogen...which is nice since it will be oil and water free).