For budget, you can go on Ebay and get most of your items from china. Many many members use these and get by well enough perhaps having to replace components more frequently than if buying from more reputable sources.
A good dependable source for ESCs is Mtroniks. They make waterproof components including ESC's, sound systems and mixers.
Howes Models do most of the parts inclusing batteries, servos, transmitters and receivers and battery chargers etc.
Action electronics do a good range of the same as Mtroniks. Their ESCs and mixers etc are not waterproof, but unless you plan to run your springer under a waterfall or in torrential rain, this is less of a problem. These come with cases that you have to drill holes in to let the leads through and a screw on lid. They also sell leads, connectors, servo controllers, fuse boards and other electrical components and have a page of diagrams showing different boats and the suggested wiring/component layouts. I am not a fan of electrickery but these are quite easy to follow.
You don't need a twelve channel transmitter. A four channel will give you ample coverage for most simple to mid complexity models. Once you start looking at controlling monitors (Fire fighting appliances) winches and revolving radar etc you start to need more channels. For the receiver, you can power it from it's own battery pack (usually 4.8 volts) or some ESCs have a Battery elimination circuit that you can plug your reciever into saving on an extra battery and thus cost. Look for 'BECC' or 'BEC' in the blurb on ESCs.
As for cost, things like the Transmitter can be used for numerous models, you just buy another receiver for the new model as you need it making it more cost effective over time. One word of warning though is that occasionally a company will discontinue a transmitter. Planet is and was extremely popular, offering 5 and 7 channel models with matching receivers, but then last year, they suddenly stopped making them leaving us with transmitters with use limited to the number of receivers we have. I have two RX*, and hope to be able to get another couple for future projects, otherwise I will have to buy from another brand.
Futaba and Spktrum are two well known and trusted brands offering various models of TX* and RX*.
If trying the China route, I would still get a good RX/TX package as even if you suffer multiple ESC failures the Transmitter should still be good for years.
Motors are available from loads of places, from your local car boot sale to the above vendors and Ebay etc. Model Motors Direct specialise n motors for models and small applications.
I reccomend reading some of the Springer build logs on this mega forum as you will see many ideas for set ups as well as other ideas, and you will get far more wisdom than in my gibberings:O)
*RX = Receiver
*TX = Transmitter
ESC = Electronic Speed Controller as opposed to older (but still popular systems) such as Bob's Boards.
