Sometimes it can be a personal call when deciding whether to go the full hog. As the folks have already mentioned, to make a clear judgement on the state of and installation requires a site visit and a proper inspection. It could be the case that it is more economical in the long run to just re-wire. It can be a bit like fixing up an old car, where you can either patch up the old body work and salvage the good or you just have the lot done in one hit.
Personally if it was my house and I could afford to I'd have it re-wired before I did anything else. This way you can specify the whole install to suit your needs for now and maybe the future. You can get all the mess done and out of the way. You are also safe in the knowledge that the install will likely last most of your life (assuming you chose to live there for that long) but even so it, also adds value to the property and most importantly you can sleep at night knowing that you and your family are safe.
If money is tight and it is possible I'd probably have the work done in stages. This is a pain and will cost you more but at least it takes the pressure off getting all the finances in place in one go. If you really want to save money then the only option is to just get done what is essentially required to make the existing install safe and up to date (assuming this is an option).
At the end of the day as the home owner its your choice. My advise - get a couple of quotes in with a couple of options and then make decision.