2000 Toyota Echo catalytic converter sensors and problems w

2000 Toyota Echo 250000 miles

I drive a 2000 Toyota Echo.

So I had the check engine light come on during a long trip. The mechanic ran the codes and told me the problem was that the sensors for the catalitic converter (pre and post) were overdue to be replaced. (apparently this should have been done 100 000 km ago.)

So he told me I would be fine to finish driving my trip and I could get the sensors replaced later...

Since then I have had trouble starting the car but it starts up if I apply the gas....

Canadian Tire quoted me $250 each plus maintenance costs. I looked online and found these two other options:

http://www.autopartsway.ca/autoparts2/index.cfm?fetch=part~ID=2000~Toyota~Echo~240666

AND

http://shop.ebay.ca/i.html?_nkw=02+sensors+toyota+echo+2000+&_sacat=0&_dmd=2&_odkw=02+sensors+toyota+echo+2000+pre&_osacat=0&bkBtn=&_trksid=p3286.m270.l1313


The ones on the Ebay site seem to be universal and cheaper ($80 each), whereas the ones on the autopartsway.ca seem to be specific (one for front one for back) and cost about $150 each

My questions, What should I buy? Should I take it to Toyota? Will a regular mechanic do the work if I arrive with my own parts in hand?
Is the starter/gas problem associated with the sensors or might this be another problem altogether?

I want this problem fixed for as little $ as possible.