One axis, multiple axes.
The electronics build may be hard
If you buy a controller board, you'll be doing electrical connections only. I.E. wires from the buttons/switchs/potentiometers/etc... to the controller.
You may need to learn how to solder, but if your neighbour helps you out I'm sure you'll get the hang of it.
I want to add common axis that will be used over most sims and I can see maybe 8 axis (this is not a fixed figure) on top of existing X52. Also of course some switches. Would a Teensy 2++ be something to look at?
I've personally not used it, but
MMJoy(2) is a feature-rich firmware package to turn a number of "development" boards, like Arduino/Teensy, into joystick controllers. They do require a few steps to make functional.
If you want a true plug-and-play solution,
Leo Bodnar makes boards that support HAL sensors and rotary encoders. They're not cheap, but they're robust, and easy to use.
Will new axis conflict with x52 axis?
They won't conflict in that they don't modify the x52. Both devices will work independently.
If you were using old software that only supports a single controller, you'd have to combine the two, but as CLOD, ROF, and DCS all support multiple controllers you shouldn't have to worry about it.
Will disconnect and reconnect of board be an issue?
It depends on the software(simulator/game), but DCS at least keeps proper track of different devices. Unless you use multiple devices with the same USB ID, it should work.
What type of switches are generally used - SPST? What would you use for replicating Spit Flap up and flap down as against a toggle switch (I assume most switches will be just off/on)?
Switches have a number of markings:
SP or DP stand for Single Pole or Double Pole, respectively. This means the amount of different electrical circuits it can complete. For Double Pole two separate circuits are present and switched, for Single Pole only one. Generally you only need Single Pole for a joystick device.
Some toggle switches may have more than two poles to be able to switch more circuits simultaneously, but you are under no obligation to connect more than one.
ST or DT stand for Single Throw or Double Throw, respectively. This means the amount of connections per switched circuit. For Double Throw, you have 3 connections: One input and two outputs. One output is off while the other is on, and vice-versa. Single Throw means you only have one input and one output, so when the switch is operated the output is turned off or on.
The relative advantage of Double Throw switches is that they can switch two outputs in hardware, so if you need two functions on a single switch, and you can't do it in software, you can connect each of the outputs to a pin on your controller board to toggle two buttons. The disadvantage to doing this is requiring more wires, fewer switches per controller(because the switch will take up two buttons).
If you find a Double Throw switch, you're free to only connect one input and one output, and leave the rest unconnected(effectively using it like a Single Throw switch).
on-off, (on)-off, (on)-off-(on), (off)-on, etc... relate the type of physical switching action. "on" means the switch will conduct electricity in that position, "off" means it will not. any position in brackets, E.G. "(on)" means the action is momentary, I.E. it's spring loaded, and when moved to that position will automatically return to another position.
on-off is a standard Single Throw switch, on-on is a standard Double Throw switch, (on)-off-(on) will always return to the center position, so it'll only press the buttons while you're pushing or pulling the switch.
Is the PC software connection really difficult for new players(no code experience but have tinkered with SQL)?
Joystick boards connect as a DirectInput device, which is natively supported by Windows(XP and up, at least). If you don't require specific configuration or calibration, you should be able to plug in the device, start the game, and assign your buttons and axes.
Is the length of wire from board to PC an issue?
Just about all joystick boards use a USB connection, which is digital, so as long as you observe the USB specification of 5 meters for high speed devices(3m for low speed), you should be fine. If you need more distance you'll have to bridge the gap with a(powered) USB hub.
Note that long wires from potentiometers or analog-output HAL effect sensors to the controller board can and likely will introduce noise(jitter).
I have read that windows only recognises 32 keys and 8 axis. Is this per board/joystick?
As mentioned previously, your device is independent from the x52, so that's 32 buttons and 8 axes in addition to that.
In theory, DirectInput should actually support 128 buttons per device, but from what I've heard few games actually support over 32 buttons.
I would somehow need to cater for full wheel brakes on Spit to left/right brakes on other planes. Not sure how?
Are you sure your simulator of choice doesn't do this in software already? It can easily treat both brake axis as cumulative for the spit, and independent for others.