For walking, use some cheap snowshoes. Works great!
For liner, you can easily find rubber pond liner for well under 5K for a pond that size. You would be best-served just using the rubber pond liner and call it a day. It may have to be welded to make 70', but if you shrunk the pond a little, you may pull it off.
I paid too much for a 20x20 heavy rubber liner, about $800 including underlayment before I got my "real" pond. You can find lighter, cheaper material if 5 years is your goal, but not entirely fair for the next guy.
First search results:
http://www.pondboy.com/items/pond-liner/list.htmNot recommending anyone, but local shops will carry the stuff. Order a whole roll if possible to use it all. Then use the tools shown to produce a weld for sealing.
