I'm planning on buying a BDPH400 from Black&Decker, looking to get more than just painting out of it, can it be used to spray herbicides like Roundup?
Can you? Absolutely!
An HVLP sprayer will spray any liquid that doesn't corrode the components and is not too thick. Herbicides aren't generally thick.
THERE ARE HOWEVER SOME IMPORTANT PRECAUTIONS!
The force the liquid comes out at and the fan shape of the sprayer means it will cover a large area so you'll need to lay a protective tarp over any non target plants within maybe 5 or more feet of the the area you're spraying depending on how windy it is. You'll also want to keep other people and animals a very safe distance away. Possibly erect some temporary plastic walls out of plastic sheeting to prevent overspray onto your neighbor's property.
HVLP sprayers atomize the liquid into fine droplets that can linger in the air for a little while and there's a decent amount of liquid that will bounce back off of any objects you're spraying so make sure to wear protective equipment that covers your head, face, eyes, arms, legs, hands, feet, well pretty much everything and also wear a good chemical respirator. This hazmat suit should do the trick and only costs $1,699 US and the shipping is only 12 bucks!
Should you? Absolutely Not!
Invest in a good pump sprayer. They're not too expensive, certainly much cheaper than a hazmat suit, and easy to use. I use mine at least once a month during the growing season. I have a review of the 2-gallon pump sprayer I use on my website. A lot of them are crappy and break but that one is built well. A lot of times I keep it outside for long periods and haven't had any components fail.
To cover large areas use a hose connected sprayer. They have a cup that you pour the concentrated liquid in and a dial to set the dilution rate (ex 2oz per gallon). As you spray the concentrate gets sucked out of the cup and mixed with the water from the hose before spraying out of the nozzle in a fan shape.
Spraying with high pressure will cause whatever you are spraying to form droplets that will float in the air. You won't be able to control what is being sprayed. No, I would not use it for pesticides. Do you use a lot of roundup or glyphosate? When someone uses a pesticide it is usually for a 'bandaid' on a preventable problem. What do you see that needs spraying?