Your comment about old monitors is valid if the old ones that your doctor is using are CoaguChek S or, gulp, an even older CoaguChek. Those meters required code strips, and regular quality control testing to confirm the (relative) accuracy of the meter. The CoaguChek XS and the InRatio machines have quality control built into the strips. The old CoaguChek also may need cleaning, because it's possible to get blood into the area where the actual testing happens. I don't think I'd have a lot of confidence in an old CoaguChek or CoaguChek S.
The ProTime meters - even from the beginning - had quality controls built into the test 'strips' (which they call cuvettes). The three channel cuvettes ran two quality control channels, and the five channel had four. They take more blood than the CoaguChek and the InRatio machines and take a bit longer to deliver a result - but all that I've seen just seem to work (with an interesting exception that I can go into if anyone is interested). I bought mine on eBay without a prescription, but you'll probably either need a prescription to buy your cuvettes from a medical supplier or to have a doctor order the cuvettes for you. Stay away from CoaguChek S -- it's a discontinued meter that Roche won't provide strips for after next October. If you're interested in buying a meter out of pocket - and don't expect to file a claim or to get reimbursed - you might consider buying one on eBay -- it's what I did but I am NOT trying to suggest that you do the same.
(I have no affiliation with ITC, the makers of the ProTime meters, in case it seems like I'm going out of my way to support them. It's just that the ProTime meters on eBay seem to be priced less than CoaguChek XS or InRatio meters, and seem to be fairly available).