A few random thoughts:
Computers: consider keeping your Mac as a master, continuing to use DP7. You do not have to run all the VSL libraries on the same computer, and by going that route, you don't necessarily have to upgrade your sequencer. Realize that in entering the VSL sample world involves extensive editing in a high-end DAW, and what you have will work fine. Consider getting a very beefy PC slave for running VSL. The VSL team members have stated repeatedly that their libraries run better on PC. Therefore, IMO, wisdom is to get a PC to run their libraries. In any case, if you go the Cube route, you are looking at needing a powerful machine (and will need that in any case for the full libraries, and possibly MIR, etc). As it is for hobby use, a dual xeon is probably overkill, but consider at least something in the i7 3930k range. Figure 32 gigs of ram, and SSDs if at all possible.
Soundcard: You did not mention a soundcard. For what you are doing, a good soundcard is essential. Will you be doing 5.1 surround, or just stereo? Depending on the answer, a different card will be required. RME is outstanding, and their drivers (very important for latency) are a known good thing. Lynx is also highly regarded. MOTU's cards are also worth looking at, but, RME, if possible, would be strongly suggested.
Listening environment: You did not mention what sort of speakers you will be using for listening/mixing/mastering. This is another important area where skimping is not wise. However, just as important, is what sort of room will you be listening in. Is the room treated? What are its dimensions? Particularly if the room is square, or extra small, the room's dimensions could cause problems. There are some circumstances, IMO, where having a high quality pair of headphones can be a major help, when the room cannot be treated, and/or one is living in an apartment where noise is an issue, etc. By "good" consider Sennheiser's 650 or something equivalent. Treating the room, and having quality monitors for good sound reproduction would be the first choice, but that is not always possible. As for the monitors, if you have a good room, plan on budgeting at least $1,000+. There is no point long-term in getting cheap primary monitors.
Sample libraries: This is the tough one. If you see yourself doing this as only a secondary hobby, consider starting with some of the SE sound libraries. They are not expensive, and they can give you a feel as to whether or not VSL's libraries mesh with how you work. They sound good in their own right. However, if I were starting from scratch again, I would go straight to the full collections. In the long run, there is no substitute for them. Getting both the standard and extended portions are important, as the extended samples are critical when trying to get as realistic result as possible.
As for Dimension, I would strongly suggest not starting there. Especially with the strings, as only the violins are currently released, that will not work for a full string section. Dimension brass, though, could be an option. The Cube certainly would be the more complete option. If resources are more limited, key collections would include: Woodwinds I, Brass I, Percussion, and for strings, it all depends. Given your interests, either Orchestra Strings I and II, or AP Strings would be logical places to start. Note though that for added realism even in orchestral settings, the Solo Strings can be of use, and they are excellent. Another technique is to layer orcestra/chamber/solo strings, though others prefer AP strings. Layering normal and muted strings is another possibility.
Note that the Orchestra Strings are divided differently from all the other string collections. While the other collections are divided as I: normal, and II: sordinos, Orchestra Strings is divided as I: violins, violas, and II: cellos, basses. Orchestra strings does include a number of muted articulations, though not to the same degree as the dedicated sordino libraries for the other string collections.
As for downloading individual libraries, both Epic Horns, and Fanfare Trumpets could be of use to you. Another option would be to consider using SE 2 for secondary instruments, though in the long run adding Woodwinds II could be a strong consideration.
Software: As for software, strongly consider MIR Pro, or at least MIR 24. I use the older MIR SE (don't have the computer power to really handle full MIR Pro) and I find MIR to be one of the most critical pieces of software I own. It greatly simplifies mixing/mastering, and it is outstanding. Although I do not yet own it, Vienna Suite is well worth looking at. When using VSL sample libraries, it makes sense to use effects plugins that are designed to be used with those samples. MIR Pro/24 does require VE Pro, and as for VI Pro 2, that too, is critical (as you mention).
One suggestion regarding VI Pro 2: As whatever you create in the free VI easily will integrate into VI Pro 2, consider learning the free VI first. There is a learning curve with VSL's software, and VI Pro 2 has many features. By getting familiar with the free VI first, it could help ease your way into VI Pro 2, and has the benefit of not involving any additional cost.
Above all though, to buy cheap is to buy twice, and seeking to spend wisely will help to avoid wasting money on items one ends up seldom using. Good luck with your decision process.