I’ll buy one when I have the money.
I’d buy one if I had the money.
The difference is the first uses future tense suggesting it is an eventuality. It will happen according to the speaker.
The second is using conditional (literally "I would buy one if I had the money"). When the condition by the if clause is met (if I had the money), the rest is satisfied (I would buy one). This is most certainly not an eventuality as it depends on the condition being true first.
I wish to see you.
I wish I could see you.
In the first sentence, "wish" is synonymous with "want". In other words, "I want to see you." It isn't a direct command, but it is still implied that you're being called by the speaker. Example:
After Bobby misbehaved in class, the teacher told him, "I wish to see you after class."
Interestingly, it can also be used in the context of "wish" being desire. In other words, "I long to see you." The correct interpretation of the first sentence is subject to tone and context. If I'm taking the second sentence into consideration, I would say that in the first sentence, "wish" means "desire" here. Example:
Mark shakes his head and sighs into the telephone, "I wish to see you one last time, before I go on my business trip."
The second sentence, "wish" here can only be interpreted as "desire", since you can only want something (transitive) and this is being used intransitively. Therefore the only interpretation is "I long to be able to see you" (unlike the former).