I would recommend a roller pump for herbicides and any general liquid pesticide but that is about it. I would go with a diaphragm pump although it is significantly more expensive. The reason why is because since you are trying to go for a multi use sprayer I am assuming you want to also use a wide array of pesticide formulations from wettable powders, emulsifieds, etc. and roller pumps wont last long with some of these formulations due to the abrasiveness. Also the diaphragm pump will create more than enough pressure to allow you to due chemigation/fertigation into your dripline a big plus nowadays for those who use drip.
Pressure is not something that you really need unless you are doing foliar fertilizing, this is because of the drift issues that come along with high pressure which can equate to fines, unintentional drift onto nearby non registered crops, and bad publicity for your farm. Therefore gather a list of general pesticides you plan on using and ask a local PCA (pest control advisor can usually be found at any chemical dealer), your county extension office, local Ag university, and/or just google search as to what flow rates, nozzle type (ie: fan, hollow cone, etc.) that you need to get best coverage of a particular crop all while minimizing the risk of drift.
Once you get the type of nozzles then go ahead and see how they fan out and then take that measurement to determine the total number of nozzles you will need. Then determine your required pressures and flow rate that you will need for the nozzle's associated spec chart.
Another note, is you can get rollover nozzle bodies that allow you to do different rates (nozzles) and turn off those you dont need , probably a plus for what you are trying to do.
