If I understand what you're asking...It's only been very recently that people 'bought' a garden by buying bags of soil. Throughout history people have added compost and manure to soil to increase its fertility. You don't have to buy soil if you have good soil or you can make your poor soil better. Scotts/Miracle Gro Corp. would like you to think their products are required to grow plants, so you will buy them. Start a compost pile if you haven't already. ;-]

firegrip Pine mulch and manure are humus. Humus is simply the residual organic matter, the OM that has not been digested by the Soil Food Web, yet. "Topsoil" is simply the top 4 to 6 inches of soil from someplace and it might be worth the price or it might not be. What do you expect that "topsoil" to be? Never accept what the seller of "topsoil" says what they are selling is but define what you want and look for that.

I suggest that you buy your soiless potting mix now and store it in your garage or basement. Finding potting mix in winter months can be difficult and I can remember others posting about having to ask for someone to go out in the snow to get bags for them. I have found that nurseries will often have some bags inside though they may be more expensive than what one can get now from the big box stores. If it freezes in your garage be sure to bring a bag inside a few days before you need it so that it can thaw. For weeds in compost: my compost pile doesn't get hot enough to kill all the seeds and I also have had seeds sprout in the compost. Mulching with shredded leaves after planting a new bed, or on an empty bed waiting for seedlings next year, will keep those weed seeds from germinating. The mulch will also keep the soil loose and the few weed seedlings that might sprout are then easily pulled while still small.

I wouldn't remove any soil at all. Clay is actually filled with tons of beneficial micronutrients. Instead of topsoil I would purchase compost and till that in to your existing soil. You will get much more for your money this way. There is no standard for topsoil quality. The only true definition for top soil is the top layer of the soil which is presumably richer and contains more organic matter than lower layers. I've got clay in some parts of my yard that could almost be turned into pottery. I'd never dig it out and replace it though. It's just not worth the effort or cost when all it really needs is a little amending.