It takes surprisingly little to grow weed outdoors. You do really need nutrients, plants won't grow for long without them (don't try getting soil with nutrients in it either, they are usually far too strong for Mary J).
You can get liquid ph testers for a few quid online, don't bother with the ones with a probe you stick in the soil, they are nearly all worthless. You do really need to know the ph of the soil and the water/nutes you are giving the plants, as without the correct ph balance, nutrients won't be absorbed properly and the growth will be poor.
You can grow with cheap soil, I flush mine out thoroughly before using to get rid of any shite they may have put in it, and mix it roughly 50-50 with perlite (also from B&Q) for better drainage and aeration of the roots.
Have a look at the growfaq, top left on this website, should steer you right.