Let me just echo what everyone else is saying. In this time period, stone building is a big deal. There's nothing simple about it. If they scavenge the stone from an older structure, that's possible.
Monasteries during this time were big, big business. Even a small one, in a small town is a way to generate income, as:
- a religious retreat for the wealthy
- a place where beer was produced
- a place where preserves were produced
- a place where books were copied/preserved
A monastery always aimed to be self-sufficient. What that means is that they produce enough goods and and animals there, on site so that they need nothing else ( or close to it), in order to limit their contact with the outside world.
The buildings and areas you're proposing aren't really in line with what a monastery really was for the time--and that even when they were ostensibly "poor" and "simple" they were actually a product of infrastructure and donations from nobility.
And you likely do need a small pond stocked with fish in their grange, if you are following the Catholic no meat Fridays rule.
You're talking about a well-stocked valley with decent resources for population, so you may want to rethink the whole "simple" thing. Because this area isn't poor or humble. They have access to a lot of food sources. For Medieval times, they'd be doing well, actually.
If you've researched the more complex structures and have a timeline on that, go with that timeline.
Let's look at each structure, because, I can tell you that it's pretty likely to be series of buildings rather than one big building, mainly because of fire:
There won't be a basement. At the very least, it won't be under any building if you are going to the least expense. It will be a root cellar, or a series of cellars dug out on their own. It's going to be shallow and small, because you don't want the trouble of hitting the water table (and these things tend to flood in rainy times) and building the supports is an engineering challenge the larger it is. When I say small, I mean closet sized. And it's unlikely to even have stairs down. Think, hole in the ground with a door on. If there's small hills, I would just build them into the side of a hill--that solves the water table problem at least, though it will still flood a bit during rainy times.
Even a one room building that fits, say, 20 people total for mass--it's going to be built respectfully and well. That's going to take time. This structure is the one most likely to be built to last, with stone.
build it out of wood. if this is early enough in the medieval period it might have a hole in the roof, in the center over the fire pit, because there are no chimneys... BTW livestock is unlikely to get a separate room, especially if you're going for humble...
Build a shelter, don't bother with most or any of the walls, just posts to tie things up and a roof to keep the rain out. Pretty fancy though, having them in their own area. But, then, monks in general had a better standard of living...
build it out of wood, might be in the same room as sleeping quarters. If you want another room, build another building.
How long will it take to build
I am going to start with the assumption that the stones for construction have magically appeared. Because quarrying and transport is a whole other thing. Based on the small sizes of things, my lowest estimate without that is about 6 months-2 years, depending on how much help they have. They will still need wooden large beam structures (if you are going with the rooftop being thatched), and those have to be cut and cured. A real-deal church and grounds at this time takes much, much longer, more like 20 years. If you want them to be proud of their tiny chapel, have it take 5-10 years and make it all stone.
It should be wood though...because stone is hella expensive. Stone is the upgraded version. You want simple and poor--it's made of wood. If, locally, there's a good source of stone, or that's the industry of the nearby lord, I can see it being donated, and it's something that they are proud of...If stone is really available and common for the area, it's still got to be shaped a bit and stacked. If they are common, and more easy to work with than wood (which in mountain areas can be true) then everything in the area will be made from stone.