Nobody is obliged to bring an old house up to modern standards. If they were, Hampton Court would have been demolished and rebuilt with cavity walls and plastic windows. So you are not obliged to change everything just because it is "not to current standards."
A crackling switch is probably just worn out (spring weak) and can easily be replaced with new. The same with sockets, especially if they show any sign of brown discolouration.
A 1930's house is pretty sure to have been rewired by now, most likely with square-pin sockets and PVC cable. If it was done prior to 1970-ish, the lighting circuits may have no earth connection, so for safety you must only use plastic (not metal) switches and light fittings. Earthing, main bonds and supplementary bonding are probably poor and should be upgraded (fairly simple).
The main problem I anticipate is that you will have too few socket outlets, and may be tempted to use adaptors and extensions. Trailing multi-way sockets are quite good in, say a bedroom or round the TV, where the load is very low. You should not use adaptors or extensions for a washer, drier or dishwasher. If you have an immersion heater the switch is likely to be brittle, so are old light fittings as old filament (incandescent) bulbs used to get very hot. Again they are cheap and easy to change.
Any electrician must (should) be a member of a self-certification scheme (and not be just a "Domestic Installer" which is the lowest grade of qualification). Look on one of the Competent Person websites, which maintain a register of qualified people, searchable by postcode. NICEIC is one example.
Beware of people who offer free surveys. Obviously they have to make a profit and recover the cost of all their free surveys by charging well for any work they find necessary.
An experienced electrician will often know within two minutes of walking through the door if you need a full rewire. But unless it is that bad, it should take about half a day to test and inspect. You want a written report of findings and recommendations. You should expect to pay the correct rate. You can ask for the cost to be allowed against the cost of remedial work that is done shortly afterwards by the same person (ask in advance).
If you have a new consumer unit, all the circuits connected to it will have to be brought up to standard. This may be expensive. I would suggest having a new CU, with RCDs or preferably RCBOs, which is big enough for the whole house, but only connect to it new work, such as a new kitchen circuit, or a new electric shower, or new outdoor lights. That way it is possible to upgrade the installation piecemeal. A recent change requires all new consumer units to be non-combustible (e.g. metal). If anyone offers to sell you a plastic one, show him the door, but you can carry on using an existing one.