You should consider a TBI system if you are trying to keep costs to an absolute minimum. TBI systems are very common in junkyards today and are relatively easy to retrofit to older engines.
TBI system do not require a cam sensor and a complete fuel and ignition system may be scavenged from even more modern cars. The size of the TBI injectors is important and you should try to find a donor car with a displacement similar to yours. Ideally, the donor car should be beside the car you are retrofitting so as that small parts you may have overlooked stripping off are still readily available. In selecting your donor car, you should pay attention to the total height of the TBI with its air cleaner as the new one may be significantly taller than your old carburetor. You should also make sure that that the TBI base will fit on your intake manifold. This will likely require the fabrication of an adapter, which add somewhat to the total installed height.
Also, as the OEM ECU is not programmed for your vehicle and will likely be a speed density system, you will need to have it reprogrammed. This adds somewhat to the installed cost and will be a recurring cost if you keep making changes to your set up. An aftermarket ECU should be considered mainly for its ability to be programmed with your own laptop computer. However, OEM ECU reprogrammers are relatively easy to find so using such an ECU is a viable option.
As the heart of the TBI system is the throttle body, you need to find one that is suitable to the type of intake manifold you are using. Generally, any throttle position sensor (TPS) provides the ECU with the same type of signal so the important consideration is how well the throttle body's throttle bores line up with those on the intake manifold. You will likely need to fabricate an adapter to mate the throttle body to your intake manifold. Adapters may be already commercially available if you have a common engine like a Chevrolet small block. Check the fuel injection products page here to see if we already have one available for you. If not, we can easily design one.