How realistic is an attack on Taiwan by China?
Obviously there is a preventive military buildup by the Taiwan fearing a military invasion.
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tl;dr: An invasion of ROC (Taiwan) by PRC (Mainland China) is highly unlikely.
In order to invade the ROC, the PRC needs two important things: 1) lift capacity to move across the strait to Taiwan, 2) force protection capacity to protect any lift efforts to cross the strait.
The PRC must take into account a few points: 1) the U.S. is staunchly opposed to a forceful revision of the status quo, regardless of who initiates it, and has proven willing to deploy up to 2 carrier groups to protect the status quo. 2) Japan is not strictly speaking allied with the ROC, but has significant interest in not seeing the relationship between the ROC and PRC change. 3) The ROC, the U.S., and Japan are all major trading partners with the PRC. 4) The PRC holds approximately $1 Trillion of U.S. federal debt, while also carrying enormous balances at the provincial and municipal level, which it generally services through Hong Kong. 5) The PRC is not currently staged for an invasion
The PRC does not have the capacity to invade the ROC, in any meaningful way. The Order of Battle shown by Global Security lists shows that China only has 52 amphibious warfare ships (the boats you stage an invasion from), almost half of which are over 20 years old. It also only has 105 active (with 200 inactive) landing craft, or how you get from the ships to the shore, meaning that the PLAN could only land about 2,000 people per wave, at best. It is well over 100 km across the strait from the mainland to Taiwan, meaning that landing craft cannot make it without staging in an amphibious warfare craft first--unlike, for example, Operation Overlord in WWII, where the landing craft could ferry straight over. Even an unopposed landing with such equipment would be difficult, and it is for this reason that the ROC keeps a large military ready to repel invasion. The large distance means that the entire landing force must be defended at sea in order to succeed.
It is for this reason that force protection becomes critical, because the loss of even one Amphibious Warfare Ship would make the invasion nearly impossible, and as little as a 10% attrition rate would be a total failure, certainly. China has very little offensive capacity for defending their lift assets from the ROC, the U.S., or Japan. Although the Chinese Air Force and Navy are modernizing, it possesses mostly older aircraft, and mostly smaller ships or submarines. Few of the submarines are a match for the destroyers and helicopters the U.S., Japan, and to a much lesser extent the ROC would use to oppose any invasion. Two U.S. carrier groups are very likely to be able to secure the strait and establish air superiority, and even if the Chinese were able to sink both carriers, the aircraft would just relocate to Taiwan, effectively tripling the Taiwanese Air Force's size. Even if the U.S. did not get involved in the war, it is possible that Japan--which has 6 Arleigh Burke-class destroyers, and their corresponding AEGIS missile systems, which would be nearly in surmountable for the entire PLAN, at this time--would intervene in its own self interest. Importantly, because any amphibious operation would involve the relocation of many assets to the Straight, all three countries would have several weeks warning to prepare themselves. Taiwan might have to fight on its own for a short while, but not several months.
Setting aside the military unlikelihood of a successful invasion, the costs of the invasion would be astronomical. The PRC still relies heavily upon exports (possibly even more so after the Shanghai market crash that is ongoing) and the major markets for this are the U.S., Japan and the ROC, which would surely embargo any goods out of China if war broke out, costing the PRC billions in revenue daily. Importantly, it is standard practice to abrogate any debt between belligerents, and the Chinese hold $1 trillion in U.S. debt, which would go up in smoke at the outbreak of war. Additionally, China's cities and provinces have spent colossal sums of money, which they have been able to borrow, because Hong Kong is a center of international finance in Asia, but that role would undoubtedly disappear if war broke out, causing massive default throughout China.
None of this is to say that war is impossible, but it is very unlikely to succeed if attempted. Occasionally, there are issues which governments believe to outweigh the economic and military costs, such as national pride and prestige, leading to wars which are prima facie unlikely to succeed. Furthermore, there are things which could change the calculation in the future. The PRC could improve its amphibious capabilities, although that would take years, or stage what it has closer to Taiwan to shorten the warning. If Taiwan initiates the crisis, the U.S. might not intervene. Japan is not obligated by treaty to participate regardless, so a shift in government in Japan to one that is less nationalistic, and more traditional in its view of the Japanese military might make it much less likely to become involved. Taiwan could decrease its military capacity meaning that a swift, small invasion becomes more likely to succeed. In the longer term, and more unlikely, the U.S. could withdraw from Asia.
If the U.S. and Japan did not become involved, it is likely that the invasion would be a massively destructive and bloody slog, where the outcome would be very much in doubt until the end, and the economic costs would certainly occur, nonetheless. Any inkling that the U.S. is decreasing its role in Asia, consequently increases the threat to Taiwan, and for this reason they emphasize domestic defense capability. Importantly, the ROC has made it a high priority to acquire 4-6 Aegis systems, which would mean that the PRC would effectively have no hope of succeeding at an invasion in the foreseeable future, but the U.S., so far, as not approved those sales.