1
As mentioned above 23" will be fine. FullHD off course since we talking about cheapest solutions. To be honest it is most comfortable because 27" requires more eyes move and bigger table to make comfortable gap between eyes and screen(80-100cm). Also monitor have to provide good color range cause otherwise you and your client may see absolutely different picture.
You may look to second-hand monitor. I worked on second-hand Asus vh238h for 7 years 12-17 hours per day and never felt bad, that was lucky $70 purchase for sure.
But before purchasing you need to see reviews and notice good and bad sides of desired model.
