For the engine an oil filter wrench can be useful. A basic set of metric sockets if you need to adjust the alternator belt tension (there is usually 2 alternators) iirc 13mm ish (maybe 14mm or 15mm) but a decent entry set will have the right sizes. My engine is a canaline and the way the alternators are fitted means I need an extension for the socket as one of the nuts/bolts is hard to get at without.
For the gearbox the drain plug is 18mm, but I have heard that some differ so maybe others can maybe chip in here. The filler/dipstick is another one that I have heard has different sizes, the workshop manual states 15mm but I seem to think it is larger on my gearbox (also a prm 150 D2). I find having a couple of high quality adjustables gets me out of trouble most times. fwiw I've found Bahco tools to be excellent quality, maybe a bit expensive, but you only buy them once.
A shallow tray that holds over 2 litres is useful when draining the gearbox unless you wish to use a pump of some sort, I prefer to use the drain plug. Always clean the inside of the drain plug too, it has a magnet in there and it collects metal gunk. I'm assuming that the engine has an oil extraction pump fitted, if not then you will need a pump of some sort (pela or equiv).
My advice is to buy what you know you need first. One addition worth having to hand is a decent multimeter.