Hi judolf.
Welcome to our Mattress Forum. 
There is certainly a lot of information available on The Mattress Underground; I’d be happy to point you in the direction of where to start first. There are really two approaches to buying a mattress … each with it’s own advantages and disadvantages.
The first approach is to become an expert in all the different materials and methods of mattress construction that are part of every mattress. This involves knowing the differences between different materials, the variations in each different material, and how to determine the different quality levels of each material (for example the difference between “cheap” 3 lb memory foam and denser 5 lb memory foam which is more durable and performs very differently). It also involves a process which in most cases is more like pulling teeth (at least for chain stores and major brands) which is finding out the information that you need to determine quality and durability. In most cases … mainstream outlets and brands don’t even know this information and in many cases they refuse to disclose it so that comparison shopping is impossible. An hour on this site reading the overviews in the mattresses section will generally make you more of an “expert” than the vast majority of the people who sell mattresses. I highly recommend avoiding this approach and the Mattress Durability Guidelines will help you avoid most of the more common choices that end up leading to the purchase of a poor quality and value mattresses.
The second approach is to first find the better outlets in your area that either make or sell higher quality and value mattresses and that are what I call “mattress people”. these are people who already know what you otherwise would need to learn and who are committed to helping you find the most suitable mattress that fits your needs and preferences in your budget and will educate you about how to make better choices rather than “sell” you a mattress that benefits them more than it provides you with long term satisfaction. These types of outlets are generally either factory direct outlets that make their own mattresses and sell them directly to the public or smaller local sleep shops that sell smaller independent or locally manufactured brands. This article will help you recognize these types of outlets.
I normally suggest enough research to ask better questions and that will help you know when people are giving you factual information or are just selling you marketing stories that have little to do with real quality and value. A mattress is only as good as the materials inside it and the construction methods that are used to make it.
A third option for those who don’t have good quality/value outlets in their area (and surprisingly there are many areas of the country that are dominated by chain stores and have few if any outlets where I would buy a single mattress that they sell) is an online purchase. This involves doing some local testing to know the materials that you prefer and then working with a knowledgeable mattress manufacturer that is skilled at helping you make good choices over the phone based on your local testing and on the type of information (like height/weight/sleeping positions/preferences) that will affect which of the mattresses they make would be your best choice. Many of these also provide the ability to re-arrange the layers of the mattress or to make layer exchanges at a nominal cost if your choices aren’t what you were hoping for. These types of outlets make the purchase of a high quality and value mattress available to those in areas of the country where there is little to no good quality and value mattresses available locally.
I would also recommend reading up on the different sleeping positions, as different positions have different comfort and support needs. It would be a bit difficult to find a bed that sleeps well for all 3 sleeping positions and multiple sleepers so I’d reevaluate the expectations you might have regarding the ideal bed.
And also take a look over our summarized mattress shopping tutorial for extra tips.
And, if you’d like to browse through mattress retailers and manufacturers we recommend to see what is offered in your budget (a lot!) then please look through our Trusted Member listing.
Our Trusted Members going through an extensive vetting process and have to meet quality, transparency, and customer service standards to be listed here. Anything you purchase from any one of our trusted members will be good quality/value products. Most of the members also have lower budget beds so I think you are very likely to find something within your budget. All of our trusted members are friendly and would be more than happy to answer any questions you have for them! If you contact any one of them with your budget, preferences, and sleep styles, they’ll be able to help match you to a mattress.
Hoping I’ve given you some good stepping stones as you get started in your search! It will be well worth your time
Phoenix