I sympathize with all your questions, but don't have all the answers.
My H and I got married and bought our first house after college 8 years ago. We didn't spend a ton, but the things we did find, we discovered after a few years that our taste had changed pretty dramatically. Unfortunately, I am currently sitting on that couch that I now hate the color of, because it is a perfectly good couch so I am not going to replace it with a new one.
I am not saying don't buy stuff, just be aware that your tastes will likely change, so maybe don't spend a huge chunk of change on many one thing. While at the same time, not buying the absolute cheapest, only to have it fall apart.
Fr curtains specifically, I have recently had to do several in our house. I like the curtain rods with the thicker rods, and the cheapest ones I could find were at target, the RE brand I think. They periodically go on allege for about 5$ off if you can wait for a sale. For curtains, I think a heavier fabric makes them look much nicer and more expensive. I found very reasonably priced heavy velvet curtains that were lined with a blackout liner, at target, but they only came in navy and cream. The navy ones are in my sons room. I sewed on for my daughters room, and the cost of fabric plus blackout liner was more than the target curtains. Generally speaking, unless you have some amazing fabric source, or use goodwill sheets, etc, it is going to be cheaper to buy ready made curtains. For ready made curtains, in my town, target and lowes have the best ones at decent prices. JCPenneys has a lot but many of them seem old lady-ish to me, and you have to play the coupon/sale game which makes me batty.
For the most part, though, we have the mini-blinds that came with the house, and nothing else, as I slowly look for nice curtains at a low price, that are the right color/pattern/shape. It takes a while. Most people I know who have had to cover a bunch of completely bare windows have gotten either the old fashioned vinyl rolling shades (HD has them, they block light, they are cheap, and they roll up to be virtually unnoticeable once you do get curtains down the line) or those white plastic faux-wood wide slatted blinds that up close look like the 20$ walmart special that they are, but from the front of the house look like fancy wide slatted blinds. Those type people tend to leave down to be decorative, and put on every window that faces the front.
My overall strategy for furnishing and decorating is to spend the money to buy a couple things new (lamps and curtains for me). I get what I want, within reason, as those are the things that I pick out to be more noticeable patterns/colors/shapes. For my living room lamps, for example, I picked a simple lamp base but bought trendy patterned shades. If I want to replace the shades later it will be 15$ each. My couch, chair, etc are plain solid colors- those are the expensive things. A 15$ lampshade is not going to break the budget, but makes the room look "done".
For wood furniture (tables, chairs, end tables, hutch, etc) they are 100% garage sales, used furniture stores, or family hand me downs. Buying new is way too expensive, and the quality sucks compared to the older stuff.
For upholstered furniture and mattresses we buy new only. Our area has huge bedbug problems and no way am I going to risk that!
For decorative items, like to sit on a shelf and look pretty, I generally just avoid (clutter!) but if I must have something, I do one large thing. I resist the temptation of many small, less expensive, things, as they don't look as good, and several small things add up to the price of one larger thing,