1) What would be your thoughts on the fleet make-up and what could be the best choice for my next BFG purchase?
There are two general schools of thought when it comes to Space Marine fleets, the Battlebarge approach and the Strike Cruiser philosophy. Essentially these two fleets have either one or two Battlebarges and a flotilla of escorts, or a contingent of strike cruisers and escorts. They are of course not mutually exclusive, and you can quite easily use both a Battlebarge and Strike cruisers.
In any event my first suggestion is that you acquire additional Strike Cruisers. They work best in packs, and with the latest fleet list, which can be found here
, they have become quite versatile, with a variety of different weapon configurations available to them.
My next suggestion would be to obtain some Thunderhawk pieces from Forgeworld. They can of course be represented by counters just fine, but I always find it nice to have some models for ordinance.
Of course, if you're planning on going with the Imperial navy (I noticed you also have a pair of Imperial cruisers), then of course your next purchase should be more Imperial cruisers, and possibly some escorts. Even if you don't wish to pursue the Imperial navy as your fleet of choice, this might be a good idea, as it will give other people a fleet to play against you.
2) How would the marines fare against other races like Chaos, Tau, and Eldar?
Space Marines tend to be close range specialists in Battlefleet Gothic. Their main strengths are in their speed and boarding abilities, and their bombardment cannons tend to be more effective at close range. They should be fairly evenly matched against other factions, especially since their update in the aforementioned fleet list. Eldar may be more difficult, due to their superior speed and manoeuvrability as well as their powerful pulsar lances, which can deal large amounts of damage and ignore high armour values, which Space Marine ships tend to rely on for protection. Necrons will also be difficult, but they are widely regarded as an extremely powerful fleet to face with any faction. Their weaknesses are in boarding and ordinance though, so a Marine fleet should be able to inflict damage.
3) And my final question, what would be the recommended amount of ships for different point levels (500, 1000, 1500, etc)?
Unfortunately, list building is not my strength, and it is not generally ATT protocol to discuss them ('Build my list' topics tend to be frowned upon), though I have given suggestions on a starting point above.
I hope this helps.