The way I mixed my beds was to take a plastic garbage can and fill it with peat. I dumped it into the bed. Then I filled the same can with vermiculite, then compost. Then I mixed it in place by hand. Didn't take too long and didn't involve any expense or complications.

The topsoil is not suitable as compost. The purpose of the compost is to attract biology and release nutrients. Top soil is inorganic and won't do either although it will likely increase water retention.

If you can't find a good, exposed to the elements compost or 5 separate bagged sources then go with what you can find and expect to use fertilizer the first year. Any compost, once exposed to the elements becomes a 'build it and they will come' sort of thing. It becomes 'alive'. Also you can get in the habit of mulching up leaves in the fall as a winter mulch on the beds and then dig them in early spring. Makes great, humusy compost. Kitchen scraps such as coffee grounds can be sprinkled on the beds for nitrogen and eggs shells crushed for calcium.

The only reason you need 5 sources of compost is for the fertilizer value. If you intend to fertilize it isn't necessary.

Thanks Justaguy for all your help. One more question? Is there any downside of adding this bagged Topsoil to my Mels mix. It looks pretty rich. I can probably find another use for it, if it will spoil my perfect mix. But if it will help in anyway and do no harm i would like to use it.

The only sense topsoil will benefit the suggested mix is in water retention.

Will it do any harm? Unlikely.

It will be heavier than all the other ingredients, it is unlikely to contain much nutrients no matter how black it looks, but it will likely retain nutrients reasonably well. It will be of very small particle size which means less oxygen to the roots, but if the bed is to be open bottomed it really doesn't matter.

Use it if you have it, but do try to keep in mind one of the 'secrets' behind the method is a light weight, moisture and nutrient retentive growing medium and soil doesn't meet the first criteria while it does the other 2. Compost, peat and vermiculite meet all 3 criteria.

The thing to understand is compost is the most important part of Mel's mix. Mel says so in his book. Mel also teaches his method in 3rd world nations and I am rock solid positive he isn't telling desperately poor people who can't afford to buy food to go out and try to buy peat and vermiculite to grow food.