As I understood you are planning to use a blockchain as a database to keep the records regarding lands, vehicles, government services etc. to avoid making illegal changes by the government officials themselves or some other third party.
To get the advantage of having a distributed database, in an application like yours especially, the blockchain should be a public one. (Rather than few government institutes running full nodes ). In that case as per your need of only protecting records it might probably be a separate blockchain for the particular purpose. And probably there should be an advantage for the participants for them to participate in mining consuming their resources. I think then you have to come up with a economic model as well. (although if this is not the exact way you are gonna do there have to be reason for ppl to participate somehow)
And regarding how secure the system will be; that depends on your model. If it's going to be a blockchain with few nodes or even let's say public with a large no of nodes and if it's still worth the money to spend on attacking the network and acquiring one or two lands there can be situations that the network may under go 51% attack or a selfish mining attack.
So what I can suggest is to study more on blockchain, consensus algorithms used, how existing blockchains achieve the data security etc. and think about a model (may be an economic model) that the miners get an advantage by running the node, and the cost to attack the system outweigh the advantages that can be gained by attacking etc.