Replacing a very old cartridge is a good idea since the suspension is likely to be very tired. However, there are various theories and opinions about how to apportion your spending for an analog setup. One viewpoint is to spend in approximately the following ratios, 30% for the table, 30% for the tonearm, 30% for the phono stage and 10% for the cartridge. When you consider the cost of a current model Planer 3 with tonearm, a $500 to $800 cart seems to be way over the top and may not provide the best bang for the buck..
While I am not an expert, I suggest that you spend a lot less on the cartridge and upgrade elsewhere. There are many upgrades out there for Rega tables, better belt, better power supply, better bearing/sub-platter, better main platter. Take a look at the Groove Tracer and Tango Spinner sites.
Another thing to consider is that for moving magnet carts the Accurus P10 has a fixed capacitance of 270 pf. If you assume that the phono cable capacitance is somewhere from 50 to 100pf, then your total will be 320 to 370 pf. Picking a cart that wants to see a low total capacitance in the 0 to 200 range will be a bit of a mismatch that can result in a lot of brightness. One older phono preamp that is very adjustable and upgradable is the Jolida JD9 which is a tube hybrid (solid state input and tube output). It can be bought used for about $400.