I'm having a go at making a two storey starter home to budget (ie less than $20,000)
Needless to say, I'm finding it a challenge!
What do you think the minimum reasonable furnishings are?

Currently I have for a total of $16,778
Cooker, fridge, sink, two bar counters + stools, trash can.
Loveseat, tv & stand.
Bed.
Shower, sink, toilet.
5 lights.
2 plants.

Most of the money has been spent on the structure of the house and decor.

If it helps, this is the house;

Outdoor trashcan is relatively important in TS4. Otherwise they'll walk all across the area in order to dump their trash into the nearest community bin. (Maybe not in the area where your screenshot was taken, since I think that has no community bins)

Also, planter + bookcase or a computer -- they can't buy books or seeds otherwise.

Thank you, that's a very good point.

edit: responding to second part, I think I may need to have a rethink!

If the budget is stretched I would leave out the TV and sink in the bathroom. The outdoor trashcan is important, in indoor one can be ditched. A bookcase and computer can be bought later.