We started with rental guns and surplus DPM from Ebay..
We got lucky when Gun-Ho closed down. On the last Skirmish they sold off their ex rental gear and we got a couple of Classic Army M15A4s for £25 that I have been refurbing as those who have followed the appropriate thread will know
Airsoft seems to be expensive at first glance but compares well with other outdoor sports like fishing, archery, mountainbiking etc.