second that on the sankens, I use two in my kit and they mix pretty well with the boom audio i take, the stock me-2's are pretty horrible, ah well i have 1 for a backup (actor sat on my other one....).
Yes, you should get a dedicated sound guy for your work, and it SHOULD be factored into your budget, but, sometimes, and especially around where I live, there is simply a shortage of them, and those that remain charge through the nose because there is such a shortage of them, anyways, in that sort of situation it's a bit of a "well... what now..." thing. Anyway, to cut it short, factor in a soundie
Although you should boom as much as you can (I just boom by default), lav mics can be life savers in situations where there isn't a way to position yourself to effectively capture dialogue within the mics pickup pattern, but, if you aren't familiar with their use they can be an absolute bitch (which is why you should boom by default - also i wouldn't capture room tone with a lav mic either).
You should read up and watch some demonstration videos on how to effectively use lav mics if you really want to go it alone, and then test them, before rocking up to set. What I normally do is wrap the microphone in moleskin (the soft furry stuff you get for your insoles at chemists) and then hide it against the actors body using medical tape (the stuff that doesnt hurt when you take it off) making sure you loop a bit of slack mic side. try to avoid using synthetic, silky and scratchy material for ur actors wardrobe.
Also, it wouldn't hurt invest in a better field recorder, if ur super serious.