Hi Anton,
It's a little difficult for anyone else to assess your needs. Any light meter you get will do the same primary function, which is to measure incident and reflected, flash and ambient light. They all do that.
Beyond that you get into features, size, and convenience.
The entire line of Sekonic meters are excellent, with the L-308S being the most basic and least expensive, with the L-758DR as the top of the line, with all the bells and whistles, and carrying a hefty price tag.
My reason for getting the L-758DR was not to impress anyone or to say I bought the best, and I had to save my money for it just as others do, so it wasn't just a "because I can" or "because I feel like it" decision. I had an L-358 and liked it very much but there were some features and accessories I wanted to add and the cost of those accessories would have brought my total cost up, approaching the cost of the L-758DR.
The L-55R is every bit as good as the L-758DR and if you can find a used one it's a great choice. The key feature that is not in the L-558R that is in the L-758DR is the ability to do custom profiling. Without getting into a whole big thing about profiling I can honestly tell you that it's not necessary and unless you already know you need it for a very specific purpose of reason then skip it, save the money and go for a used L-558R.
The L-308S would work fine. It's a simple meter with no provision for add-on spot metering attachments or wireless triggering via Pocket Wizards. If that's not important or worth the cost to you then then by all means save the money and get the L-308S.
The L-358 is one of the most popular meters being sold today. You can add the Pocket Wizard module for radio triggering but the spot metering attachments are rather expensive. If you know you want spot metering then the L-558R is a likely candidate. It has built in spot metering and a Pocket Wizard transmitter module and it's an outstanding meter.
The L-758DR has a huge feature set and lots of cool and useful features, but it's not for everyone. It's still a light meter and ultimately it's not going to give you better exposures than with other meters. It will display a lot of information and profiling proves useful is you're doing high ISO ambient light shooting or if you're shooting JPG and perhaps going right to print or proof sheet for a customer or client. Beyond that, having custom profiling is a feature that most people simply don't need.
The L-558R is not in production but if you can find one, it's superb and you would not be disappointed.
