Filling up beds growing in soil/compost

Ok I told you all I was new and totally inexperienced to gardening

I have bought a new green house 6x10
I hope to grow tomatoes, and bring on seeds to bedding plants for the spring etc
nothing exotic

Now my plan is to make the beds with corrugated iron 2' 6" deep 2' 6" wide and 6' long

I intended to fill these with cheap "pay less" compost bags, and a couple of inches of quality compost bags for the top layer about 6"
Expensive to start out but I hope that given nutrients over the years to come it will be good
Is this the way to go, or should I just half fill with garden soil /clays then top up with good mirical gro or similar
(I would have about three inches of pebbles/stone chips in the bottom as drainage)

Just what is the thing to do in reality

I am not rich, and it would be expensive, but over the years I have found to buy once is far cheaper in the long run, and easier than starting over again until it is right
(I do have a full 5' diameter x 3' high home compost bin, waste vegetable matter, Plant pot soil, and grass cuttings about two years old but it is just a mushy wet sludge I even went hunting and chucked a dozen worms in for good measure, but can not see any now )

Thanks in anticipation
Regards Jack McH

 

I'd agree with JWK, some top soil in there will help keep the moisture locked in, as organic based compost can dry out a lot quicker.
In my green house beds I've got a mix of top soil, multipurpose compost and rotted horse manure. So far the tomatoes and cucumbers are loving it.