Is there a device that can be relied upon...?
Try e-bay, you should be able to get a good price for both machines.
Sorry to be flippant, but home measurement is unlikely to make a difference to either the management of your blood pressure, or the long term outcomes.
Try and obtain 3 or 4 results from a calibrated device (and operator), over a period of time, get your cholesterol checked if worried, and chat with your gp.
There are only 3 things that alter (or delay?) the risk of you dying from an atherosclerotic event; long term blood pressure control, lipid management and stopping smoking.
Anything else is just fudging 'round the edges, and are subject to the law's of diminishing returns, unfortunately.