Looking for guidance and designs for marine application.

You have to decide if you want a "plug and play" self regulating system (such as an all Victron charge controller, battery, inverter system) - quick installation and lots of support available, or a home brew DIY system (individual components) - cheaper, but riskier unless you know what you are doing and more demanding of knowledge, maintenance and care in operations. Better to spend a bit more and be safe than to cut corners on a boat (especially if you aren't technically inclined). $2000 is a bit on the low end for a useful, safe self regulating system, unless you are just aiming a running a phone and lights.

Victron gear is very popular amongst the boating crowd, generally good quality stuff and it has very simple well designed bluetooth monitoring/control built in.  Most of their gear is very good value for the $ but their batteries are rather pricey (unless you can find a deal on them) so you might explore other battery options that are compatible with Victron Cerbo protocols. 

For a self regulating system, you need: panels, wiring, charge controller, inverter/charger, comm module and batteries (if you are not technical they are hopefully networkable to charge controller/inverter to take care of their state of charge etc.).  Typically, batteries are the major expense.

The boating forums such as Cruisers Forum (sailing), Hull Truth (power) etc have good advice specifically for boat solar gear.  The DIY Solar forum is an extremely active place to get general practical and technical advice on selecting gear and designing a system.

Note that marine battery systems are usually divided into "house" battery (LiFePO4 solar system for lights, household use) and lead acid starting systems.   This is because: 

1. You want to always maintain battery reserve for starting
2. Marine engine alternators are not typically designed for charging large or Li-Ion type batteries (Li batteries accept more charge, the high currents can burn out standard alternators quickly)
3. Separation allows the house battery to be of a different type (Li instead of Lead Acid) and of a more efficient, higher voltage like 24V or 48V to power the inverter. 12 V batteries and inverters are really only optimal for engine starting and not for running loads like microwaves etc. 
4. Separation simplifies grounding / shore power use etc etc.  Isolation and grounding of marine shore power chargers and inverters is a topic all of it's own, requiring some research to understand the issues.

It's much easier to estimate and add up load side Watt-hr usages of common appliances like coffee pot,  computer, TV, microwave in terms of Watt-hrs instead of going through gyrations to relate to battery side "Amp-hrs".

Battery systems are thus better looked at in terms of kW-hr or Watt-hr energy storage (Volts times amp-hrs) instead of amp-hrs alone (which doesn't really tell you how much energy is stored). 400 Amp-hrs at 12V is 4800 W-hr or 4.8 kW-hr.  In addition to energy storage capacity you need to assess what your peak load might be so as to choose an inverter (ie a typical small microwave is usually around 1300 Watts peak but you can get smaller more efficient inverter model microwaves that draw around 1000W). A 1200W 24V Victron inverter is about US$450.

You probably want a reputably branded LiFePO4 type battery system with a good BMS that has bluetooth monitoring capabilities.  There are "drop-in" 12V lithium battery that are ~ 1 to 2 kW-hr sized similar to car batteries, and you could use multiple batteries in parallel to get your desired capacity. However, having multiple batteries that are not designed for that or that are not networked together via comms can be an invitation to state of charge imbalances unless you keep a close eye on them yourself.

You may find that 5 kW-hr of storage might work out to be one or two 24 V batteries that can communicate with Victron gear through their Cerbo hub or similar (such as an Eco-Worthy battery). These batteries might be a less costly solution for your loads. Larger fewer batteries will have fewer connection points of failure or opportunities for imbalances.  Going too small with many batteries is a classic design failure. Much easier to expand your system if you start with larger batteries in the first place instead of having to make all sorts of connections to incrementally grow a system that ends up unreliable as a result of too many batteries all doing (or not doing) their own thing.

Whatever you do, make sure you incorporate quality properly sized wiring (fully tinned marine copper wire, sealed adhesive heatshrink on closed end tinned lugs with properly crimped cables), overcurrent protection on each circuit (matched to wire size to protect wiring against meltdown fires).  Also, to prevent electrolytic hull, engine or fitting corrosion (especially in salt water) make sure that all circuits are wired back to ground on battery. Unlike circuits in cars, in boats don't use anything for the ground return except dedicated ground wires and ensure that all grounds go back to a single hefty ground bus with one connection to your house battery. Keep wiring out of the bilge (pinholes in insulation are trouble)!!

Fire is a catastrophic situation on a boat so make sure that you avoid "Temu" class batteries .....!!   Renogy and similar low tier batteries are low cost, but also somewhat suspect in terms of build quality, although some people use them.