As a matter of idle curiousity , the Y axis is stated as having 300mm of movement, mine can only manage about 185mm before hitting the dowelled stops. Is this correct ?
The 'Y' travel quoted in the manual is 200mm. Mine can do 205 before the stop engages. Your travel of 185 suggests that something is interfering. Incidentally, do not try to extend the travel by removing the stop blocks. This could cause some damage if you went beyond the limit under power feed.
You can extend the useful Y tavel in two different ways.
If you are using the vertical head, the overarm gives you about an additional 240mm of travel, but you need to keep track of position yourself using slip gauges for example. The overarm travel is actually 400 mm, but you can't use all of it with the vertical head because it retracts past the horizontal head for part of its travel. A DRO makes keeping track easy.
If your 159 has a tilt/tilt/swivel/slide table (most UK imports seem to) then you can also use the travel of the auxiliary slide in addition. This gives you a further 140 mm. This of course has the full dial to keep track of position. If you use this slide in this way you need to clock the table absolutely square of course.
So although the nominal Y travel is only about 200 mm, you can get up to 580 mm at the expense of a bit of complication.
One thing that you should do with a new 159 is to drain and replace the oil in the upper spindle gearbox. If the mill was used with coolant, it is possible for it to splash up and into the spindle gearbox, where it will contaminate the oil and can cause problems with the gears and bearings - and there are a lot of those. An oil change will stop this from getting worse. You might as well do the same with the lower feed gearbox, but that is not likely to be contaminated with coolant.