The two vital bits of kit, are quality boots and a good quality waterproof.
Not that base layers are a waste of money, but just about all of us have items of old clothing, that will keep us warm in differing temperatures.
Just look at the kit Alfred Wainwright used to walk the Lakeland Fells in, it served him well, and he had none of the choice us walkers have today, and he lived to tell the tail.
I know that's a daft suggestion, but kit manufactured back in the days of Wainwright era, was just as well made as today's, but was harder wearing.
Walking does not have to be an expensive hobby, but you can over spend on big named kit, when lesser brands are just as good, for far less expenditure.
High quality boots, and there are many within your good budget of £150, and £170 is more than enough cash for a good waterproof.
The important thing is to not over spend, and by the looks of things you have a healthy budget.
There's nothing wrong with the gear sold in Aldis, with some of the base layers superb value for money, and probably just as hard wearing as gear costing considerably more.
I like your idea on Montane clothing, its high quality stuff, and their reliance on Pertex is a great choice.
What you wear underneath the waterproof is totally optional, and remember this years expensive base layer will be replaced by equally expensive new kit next season.
Some of us MUST be seen in the right branded kit, whilst others are not too picky, and it does not always come down to budget.
Aldi has sold some incredible kit over the years, and even though their not renouned for selling big named brands, it serves the purpose well, and should not be ignored just because its at a bargain price.
Visit a good walking shop, and invest most of your cash on the boots and the waterproof, the rest of the kit can be sourced elsewhere, and if your not too bothered about the brand name on your chest, huge savings can be made.
Comfortable feet are essential, and staying dry in adverse weather stands to reason.
Good luck, and stick rigidly to your set budget, and do not be persuaded by shop staff to buy gear that is over your budget.
That's easier said than done, but £170 will buy you one heck of a good jacket, one that will last many years.