Hi mate. I would say every gun has its own purpose. Although you can do all jobs with each of them but they will shine in areas they are supposed to be used to and will lack performance in others. Airless gun For the walls and ceiling I would definitely use airless. High pressure it pumps the paint allow it atomise correctly and you will not need to dilute it. Should be ok to cover with 2 coats of decent paint. You can cover big areas in a short time so transition lines will not have time to dry out and paint will look uniform. Downside is loos if pint if you use something expensive like Poli. Oil vase paints you will have paint loses +a bucket of white spirits to flush it out. It all depends on the size of the job. "Is the job worth the candle" HVLP: The main upside is the price for setup. No losses of paint and you can work with small items. Downside is that it will take years to paint big areas and transition lines will be horrible+ you'll have to dilute paint to water consistency and paint 5 times. Downside of any stray guy is that you need to mask everything and your grandma who is sitting next door and what thing countdown 😂 Small airborne particulars will get everywhere. So sometimes it's just better to go over with a roller. To roll the ceiling will take 1h. To mask everything will take 2h. Same for doors that are hung already. To mask around architrave skirting and everything else will take a few hours. So long story short. HVLP : only for furniture and use in workshops or spray kitchens on site. Airless: Job site for big projects. No furniture flooring etc. Anything else: Roller and paint brush Hope it helps 🤞

Water based paint I would aim for the same consistency. Ratios could vary depending on paint.