Go to Art Galleries and see the Leica shots from original prints. You will be attracted.
Noone reached the optical engineering mathematics and glass research knowledge as Leica.
In my belief , older the Leica better the Leica. Dont waste your money to modern Leicas , buy a series III and a Summar , Summitar , Elmar , Summaron , Hektor , Telyt and you will reach the top of RF mountain.
You will invest 1/10 price and get 10x quality.
If you dont believe me , look at how people dressed like at 30s and 70s. After WW2 , all quality dropped.
Old Leicas are made by elegant people for elegant people.
Its sad , now old Leitz sold at corners like a Potato.
You cant see the details and quality of Leica at a LCD screen also.
Before investing thousand , invest 100 dollars and buy photography books and magazines .
If you are not a human photographer , you wont use its capacity , if you use non Kodak film , you will ruin the results , I am strongly believe this.
Good to know.
"Truth and love are my law and worship. Form and conscience are my manifestation and guide. Nature and peace are my shelter and companions. Order is my attitude. Beauty and perfection are my attack."
The OP simply wants to be informed about the possible choices in an insightful way and correctly. To elevate simple things to "religious" levels would not help him in anyway.
Regarding Summitar; I have three samples, one is with straight aperture blades (older version but coated) the others are with spherical hexagonal aperture blades. Compared to the two DR-, one version 4 and one version V (w. focusing tab) Summicrons 50/2.0, the Summitars are slightly inferior at the center, grossly inferior around edges at all apertures starting from f2.0. Contrast: Summitars are lower at any aperture.
HP5: Since 1963 I have been using Ilford hi-speed films starting with the HP3, then the HP4 and finally the HP5 series besides Tri-X too. They are wonderful films.. even HCB was using the same since the introduction of the HP3 until he retired.
Regarding the ZI and MP he was considering: I am sure he would love the finder of the ZI, so bright and easy to use with eyeglasses too. However the MP shines from the quality point of view as well as mechanically. IMHO, no need to buy a new MP since slightly used ones sometimes surface with half of the price of the new.
Finely used and CLA'd M2s, M3s and M4s also could be recommendable choices for anyone who can afford as they last long years between service intervals. A Digisix or Sekonic can complement these classical models with low cost, making them attractive even for beginners.