I'm not sure I understand the question. Do you want basic mouldings now that will be ripped out later when you start adding details? If so, then I'd go with the cheapest option available and I suspect that actual moulding will be cheaper than using 1x lumber. Or are you saying you want good quality but "essential" moulding now (baseboard and door/window casing) and then add things like crown and chair at a later date?

I don't like MDF moulding because I find it difficult to cope and I don't think nails have the same holding power. But since you're not installing it... When sanded, primed, and two top coats, finger jointed pine should look just fine. Poplar is another option, it's a softer hardwood that takes paint very well.

NO, we don't want to rip out the old ones.
We'd like to put in something that we can embellish further when have more funds available.

Eg, our builder is suggestion we put in a 1x5 now, and then later add a fancy casing to the top of it, and a quarter arch to the base.

What I'm questioning/wondering is whether the 1x5 is even cost effective, or if i should just eat the cost and buy the taller base molding now.

And the builder is only installing the baseboards - we'll be doing the chair rail, crown ourselves later, hence wondering if mixing and match wood types (ldf/mdf/pine, etc) would be an aesthetic issue if painting.

Eg

What is ldf?

If you use 3/4" stock for the baseboard, your door trims will also have to be 3/4", or thicker, or you will have to use plinth blocks. My old house has 1x8 base with a 1 1/4" cap, and it does not look oversize at all to my eye. To me a 1x5 with a cap would look small, but not compared to clamshell molding. So it's all relative, and depends on what your eye is accustomed to seeing.
Casey

Or here's another question for the group:

- would simple 1/4 round be 'workable' or allowed for a temporary baseboard, to cover gaps? It's certainly cheap enough, and would give me more time/flexibility to play with the rest of the baseboard design after I've moved in.

Short answer is yes. quarter round is cheap, quick to install, and won't do any damage if you remove it later and replace it with something taller.
To be clear, you mean adding it on top of a 1x5 (?) as a very small base cap? It's far too small to use by itself in lieu of baseboard proper.
But for almost the same money you could use an ogee cap (WM 164) on a 1x6 and be done with it. nearly the same labor, much nicer look, and permanent.
Casey