Hi Rob,
Good to hear that your going to try and cut the M&T by hand. Peter Selkers uTube videos are excellent and you certainly can cut a Mortice with a standard chisel. However, it's not that easy, the benefit of a Mortice chisel is that it's thickness is the essence to its utility. Once you have cut a slot initially, the thickness of the chisel ensures that you cannot wander off line. With a standard chisel you have to guide it all the way through. You have to focus on keeping the chisel perpendicular to the wood at all times, not easy but possible. A mortice chisel is also sharpened (Tin hat on for mentioning the war!) with a curved chamfer so that it both cuts in a curve and also is far easier to rock back and dislodge the shaving that has been cut. I would buy a single 3/8" or a 8mm chisel to initially have a go with. The Narex have good reviews, alternatively there are lots of cheap second hand ones to be picked up. The size I've suggested is circa 1/3 of standard 2"X1" or 50 x 25mm which is great for lots of get you started projects. If you were to add say a 5/8" at some point you can make a lot of stuff without any further expense required.
I have learnt a lot from Jacob over the years, he has saved me from making expensive errors of judgement on a number of occasions. My recommendation, is never to take the advise of any one contribute as being fact, we all have different views and I would recommend you to read a number of threads, or views in a thread you start and make your own mind up about a subject / topic. Generally, the views tend to either coincide or form two camps, you will need to decide which fits your style of doing things best.