The choice of mower will depend on several factors
Firstly you say 'grass cutting business'
I have customers who cut mostly small areas with pedestrian mowers, but have the odd contract for larger lawns, and 'one offs' that require a ride on.
I have others who cut mostly large areas with ride ons.
The first group usually have a decent domestic machine which suits them fine, the second have a pro machine.
Obviously, cost is a factor, as is storage and transportation method.
As others have said, if in the second category, you should avoid domestic machines and go for pro machines, most will be in the 5-7k new price bracket unless considering 4wds, or triples etc.
If in the first category, which it sounds like you are, you have to be very careful in your choice. There are many so called collectors which simply won't collect damp or long grass.
The quality of many is also poor an running costs can be high.
Westwood and Countax have been mentioned, I would avoid these like the plague, they cut very poorly and are unreliable with high maintenance costs. Sure, the brush collector works well but is very cumbersome in use.
Avoid the smaller cheaper side discharge machines. They will not collect unless fitted with a side blower, which is not an option on most machines in this category.
The John Deere X155 is a good bet, it is well built and collects well. The X300, or 305 are even better built and collect very well due to their turbo fan which takes grass from the centre of the deck and blasts it into the grass box.
As Likeit says, mulchers, especially the Stiga are very good, but you do have to work with them and are probably not best suited to irregular cuts.
As others have said, find a decent machine and ask on here. Someone will have first hand experience and offer advice on that particular machine.
Hope this helps.