You and your fiancé can get married in the UK. If you just want the wedding celebrated in the UK followed by your return to Zimbabwe within 180 days, then your fiancé should apply for a Marriage Visit Visa. This will allow him/her to enter the UK as a visitor and use the MVV as valid credentials at your chosen Registry Office. You should identify the Registry Office and contact them before submitting the application.
There are no provisions in the law to switch from an MVV into settlement. You would need to leave and apply for a spouse visa. Alternatively you can apply for a fiancé visa to get married in the UK with a view to settlement (this visa will allow your wife to switch into settled status with an in-country application, but there's a breathtaking price difference).
Spouse visas are governed by Paragraph 281 of the Immigration Rules. The Fiance visa is explained in Paragraph 291 of the same rules (same link).
For your question about the financial requirements... There are no exemptions based upon age, however there is a provision allowing you to use your British pension to help your fiancé meet the requirements. All other savings and income must be in your fiancé's name. The financial hurdle is used for both spouse and fiancé visas.
This is explained in the so-called Appendix FM to the rules.
Given that your fiance is over 65, she does not need to pass the English language test.
Note that this is an election year and the financial requirements have been politically controversial. To date none of the parties have pledged to repeal the financial requirements, but it's advisable to monitor the debates and assess your chances of seeing the requirement lifted as a result of election promises.