Hi Thatg,
I would encourage you to read the tutorial post and some of the guidelines there. As you will discover … the Novaform Serafina uses low quality memory foam in the comfort layers and I would avoid it (see post #2 here as well).
I would also keep in mind that softness and firmness is very subjective and will vary with body type, sleeping position, and the sensations that each person experiences as being “soft”. What feel firm to one person may feel soft to another. This is especially true with memory foam which can vary with temperature and humidity and how much you change positions as well. The most reliable way to know what will feel soft or firm for you is either your own careful and objective testing and experience or in the case of an online purchase a more detailed conversation with a knowledgeable and experienced retailer or manufacturer that has your best interests at heart and can take into account the many variables that can affect how you feel on a mattress.
The tutorial post has a link to a list of some of the better online memory foam retailers and manufacturers I’m aware of and there are many good options available in your budget range.
Phoenix