Something that you can do to help simulate the friction/resistance of the water which helps load the rod when doing practice casting on the lawn -- build yourself a "grass-catcher leader". I read this some place and it makes sense. I found that doing spey casts on grass does not have the same feel as water since the water is somewhat adhesive and adds friction/resistance which can help or hinder your cast.
To build a grass-catcher leader simple take some mono that you would use for your leader, say 12lb or what ever you have. Do a barrel knot or blood knot every foot and leave longish tags sticking out each side of the knot. These will catch the grass and give you a similar feel to what the water will do to your line. If you make about a 10 foot leader that should do. You can fine tune it to what feels right by adjusting the total length and the length of the tag ends of each knot, but I'd leave the tags around 1/2 inch to start.
Other good things to do for just getting the feel is to make spirals with the line after making a cast that lays out well. Kinda like starting a snake roll, but then just keep spiraling the line. Obviously let the line crash or flick it someplace if it gets too dangerous. Get used to doing two loops in either direction and see if you can land your leader withing a rods length of yourself like setting up for a snake roll. Another fun thing to do is try a snap-t towards yourself and try to catch the leader with your hand. It's a snappy way to check your fly too when you are on the river and dont want to strip in all your line.
Tieflier - if you have a way underlined rod, you can also try overhead casts. It's a good way to get a feel for the flex and loading of the rod. Perhaps your local fly shop might let you demo a line or two before you buy a line - or perhaps someone here on the forum might loan you a line for a few months.
If it were me, I wouldn't hesitate at getting a scandinavian head line. Depending on the taper, it should be able to do all the dry fly fishing you want, plus you can add a poly tip or some T-14 or what ever to do your deeper work. Another consideration is getting a set-up that allows you to save some costs - buy a running line and then you can loop on any kind of head/line you want. It's most likely that you will not be using a long-belly line anytime soon, and will most likely be using a skagit or scandi head line, both of which suit the running line + head system. Plus, heads usually run between 40 and 80 bucks, about half of what a full line costs. Running line might get as high as 50 bucks, but when you do the math, you could get a running line + three different style heads for much less than three lines and you would have 99% of your fishing situations covered.
Any way, keep practicing - I gotta get my two hander on the water more...but I just love the spey casts with a single hander!
If you haven't seen this guy yet, get ready for some awesome single handed spey casts on a 9ft 5wt:
http://www.youtube.com/watch?v=i0ZJhum4_Q4&feature=fvsr
http://www.youtube.com/watch?v=0QhDavr24xA