Katie, Suncast's website has assembly instructions for what I believe to be the one you're describing. For securing it to the ground, it mentions four round recessed areas on the base that can be drilled out and 12" nails/landscape spikes driven through them into the ground. For your application with the slab, I would recommend 3"-4" masonry screws(Assuming a 4" thick slab, slightly longer depending on how thick the base is) with galvanized washers to prevent them from tearing through the base. You would need four of them and an appropriately sized masonry bit, probably 1/8" or 3/16". The screws and bit can probably be had for under $10. You can ask someone at HD what size bit would be appropriate for what masonry screws they have. As an alternative, you might try a construction adhesive such as Liquid Nails or a polyurethane adhesive. These can be bought in tubes for caulk guns for around $5 or so. I probably wouldn't use a continuous bead around the bottom, just several dabs of it where the base will contact the concrete. You could then use a scraper to separate the base from the pad if you want to either move it or purchase something bigger. Either would probably be good enough to keep your shed stable and in place. I personally favor poly adhesives. Here's a link to what I believe to be the unit you describe. Under the picture is a link for "Assembly instructions". Never hurts to be prepared. :-) Good luck! Here is a link that might be useful: Suncast Vertical Shed

The cheapest way would be to rent a jack hammer and call up a disposal company for a dumpster (make sure you tell them it's for concrete). Break up the slab, put the pieces in a wheel barrel, and move them to the dumpster. Call up the local rental place, tell them what you want to do, and ask how much it would cost to rent the equipment. Ditto for the dumpster. You could also pay someone to provide the labor for the previous method. The easiest way would be to pay someone to do it for you. Most contractors would use a skid steer (aka Bobcat) with a jack hammer attachment to break up the concrete. The tires or tracks will cause some damage to the yard. Call around and get some prices. You'll also need some dirt to fill in. Have a landscape company dump it in the driveway the same day -- the skid steer guy can fill in when he's done with removing the concrete.

I'm not sure why you have never had moisture inside the existing shed given those issues, and changing the roof line so that it increases the amount of water and snow on the uphill side sounds like a recipe for rotting of a wood structure over time. It may be that the existing roof design is a big factor in preventing water damage to date. Moisture will cause expansion and contraction of wood, which means that a caulk/adhesive bond between wood and concrete will likely fail over time, so it's hard to recommend this type of connection. I have three thoughts on these challenges: 1): excavate around the pad so you can lower the soil level to several inches below the top of the pad, and trench down and install 4" drains all around the perimeter. Then you should be able to overlap the sides of the existing pad; or 2): pour a new, slightly smaller, 4" pad on top of the existing one: an 8'X10'X4" pad will take about one yard of concrete. Then you can overlap the siding; or 3): Lay a cinder block perimeter foundation in mortar all around the outer edge of the existing pad and build your shed on top of this. With any of these approaches you should also do two things: 1): Securely anchor the shed to the foundation; and 2): Install gutters and downspouts.

What kind of soil do you have? What is the sunlight like where you want to plant (full sun, part sun, shade, dappled, morning sun/afternoon shade, etc.)? Also, I notice that slope in your yard. Does the wide area in front of the shed have good drainage or does it tend to stay wet for a while after a rain? From the image you provided, it looks like you like the aesthetics of an informal, cottage-style garden. Is this correct? If you are intent on planting this year, spend the rest of the summer planning and digging your beds, amending the soil, coming up with a list of possible plants and their locations, and that sort of thing. Then you can start the actual planting this fall and next spring (or just wait until next spring entirely). It is certainly possible to plant things in the middle of summer, but it's not the best time because the heat tends to stress transplanted plants and because you have to be really diligent about keeping up with watering needs.