I am not so sure if you can simply put non leading axle forks on it to make it into a 77 model. Normally backwards engineering like that is not accepted but just for some club days it might be ok? The frames have different steering angles too not sure on exact model when they changed though. A steel DR400/500/SP500 swingarm is going to be a 'cheap' upgrade, anything else alloy is going to be a few $100. YZ forks are common modification. It really depends on how serious you are, how much $ and time you have and how good you want it to end up. Some guys use stock swingarm and forks and then just fit decent shocks and set the forks up a bit better and thats it, and then others go all out with different swingarms, forks, triple clamps, sometimes de-raking the frame etc and really do quite a lot.
I think the general rule is for 'cheap' you wont get quality. There are a number of set ups and parts i could suggest but i dont think they could be in the 'cheap' category. What is cheap to someone may be expensive to you so its a little hard to make real suggestions at the moment. I would work out a budget on how much you want to spend on front and rear suspension and then go from there.