We're more than happy to give a helping hand.
We'll put the DFS issues to one side for now, as it'll be tricky to diagnose without the rest of the machine up and running.
Firstly, just to check, when you power up the machine, you have a long beep, and do you see anything on your monitor? A black screen, a black screen and a flashing cursor, or no video signal at all?
In terms of working out what's where in the Beeb, and what it does,
this page of the 8BS website is a really good resource: it has a picture of the mainboard that lets you click on a chip to see a description of what it does. A copy of the circuit diagram is useful when you're trying to work out what should be connected to what; there are a number of scans of original paper diagrams online, but I tend to work from
this redrawn diagram. If you want to do some in-depth reading, the
BBC Micro Service Manual has a lot of technical information on how the machine works, but don't worry if you leave that one on the shelf for now.
I'm working on the assumption that you have a multimeter, and are confident in how to use it. If I'm going too fast, let me know. (And for that matter, shout out if you have any other testing equipment, such as a logic probe or oscilloscope.)
Here are a few tests that can help us work out what is or isn't working in your Beeb at the moment:
As has been mentioned earlier, give each socketed chip a firm press, to make sure they're in securely and haven't loosened over time. Then turn on the machine and see if anything good shows onscreen.
Take a DC voltage reading of pins 4, 5, 6, 7, and 8 of IC6 (the 5C094 Video ULA Chip) with the machine switched on. Pin 8 is the master 16MHz clock signal for the board, and it's divided into 8, 4, 2, and 1MHz clock signals for the rest of the board by IC6. If these are missing, the Beeb's chips won't know when they're meant to do anything.
Incidentally, IC pins are numbered counter-clockwise from the notch or dot at one end of the chip, e.g.:
A DC voltage is effectively an averaged measurement, so the average of a good square wave clock signal that's oscillating between 0V-5V would be around 2.5V. (From memory, pin 8's voltage may be slightly different to the other four.) An oscilloscope or logic probe would give us extra information, but we can manage without for now.
Measure the DC voltage on pin 40 of IC1 (the 6502 CPU) with the machine switched on. This is the Reset input to the CPU, which tells it to get its act together and start running. Note what happens to the voltage when you press and release the Break key on the keyboard. It should be 3-5V, going down to ~0V when you press Break.
Locate jumper S25, in the North-East of the board near the RGB video output. This jumper configures how much RAM is available to the Beeb (think Model A vs Model B). It's normally in the North position for a Model B, to enable both of the 16K RAM banks. Switch the machine off, then move the jumper to the South position, and see if the machine will boot. Then try it again with S25 removed. This helps test whether there's a fault in one of the RAM chips, by disabling half of them at a time. (Put S25 back North again at the end if you don't see any change.)
Check continuity between the OS ROM and the CPU. You'll want to switch your machine off for these tests. The OS (in IC51) has sixteen address lines and eight data lines that connect it to the CPU. These can be identified from the circuit diagram:
We see from these, for example that pin 25 of IC1 and pin 1 of IC51 both correspond to line A15 of the address bus. These should be connected together electrically, and the
continuity function should give a loud beep if there is such a connection through the two probes and the circuit they're touching.
To test A15, place one probe on the shoulder of pin 25 of IC1, and the other probe on pin 1 of IC51. Putting the probes on the shoulders of the chip itself, rather than the socket, means we'll also be testing the connection between the chips and their sockets.
Repeat this process for all of the address and data pins, double-checking and noting any that don't pass the continuity test. If you have some failures, re-seating the two chips may help. Use a small flat-head screwdriver alternately under either end of the chip, and gently lever the chip up a few millimetres, before pressing it back down with your hand. Repeat the continuity test again, to see if the fault is gone.
To give you an indication (hold that wallet tight mind) go 
